Fix time spine validation error (#354)

### Description

In my last PR, I removed an early return statement
[here](https://github.com/dbt-labs/dbt-semantic-interfaces/pull/353/files#diff-23094b690ea30acfa70bbe8b29789680bc46582dfcc3c56ce1e8ce929d62781eL42).
This introduced a bug when users hadn't defined a time_spine
configuration. This bug wasn't released. I noticed it in my testing.

### Checklist

- [x] I have read [the contributing
guide](https://github.com/dbt-labs/dbt-semantic-interfaces/blob/main/CONTRIBUTING.md)
and understand what's expected of me
- [x] I have signed the
[CLA](https://docs.getdbt.com/docs/contributor-license-agreements)
- [x] This PR includes tests, or tests are not required/relevant for
this PR
- [x] I have run `changie new` to [create a changelog
entry](https://github.com/dbt-labs/dbt-semantic-interfaces/blob/main/CONTRIBUTING.md#adding-a-changelog-entry)

Author: DevonFulcher

.changes/unreleased/Fixes-20241010-142329.yaml

@@ -0,0 +1,6 @@
+kind: Fixes
+body: Fix time spine validation error
+time: 2024-10-10T14:23:29.392029-05:00
+custom:
+    Author: DevonFulcher
+    Issue: None

README.md

@@ -10,7 +10,7 @@
 
 # dbt-semantic-interfaces
 
-This repo contains the shared semantic classes, default validation, and tests designed to be used by both the dbt-core and MetricFlow projects. By centralizing these shared resources, we aim to maintain consistency and reduce code duplication across both projects. 
+This repo contains the shared semantic classes, default validation, and tests designed to be used by both the dbt-core and MetricFlow projects. By centralizing these shared resources, we aim to maintain consistency and reduce code duplication across both projects.
 
 ## Features
 - Protocols for shared semantic classes: Define the interfaces and common attributes that must be implemented by the objects in both projects.

dbt_semantic_interfaces/validations/time_spines.py

@@ -28,10 +28,14 @@ def validate_manifest(semantic_manifest: SemanticManifestT) -> Sequence[Validati
         if not semantic_manifest.semantic_models:
             return issues
 
+        time_spines = semantic_manifest.project_configuration.time_spines
+        if not time_spines:
+            return issues
+
         # Verify that there is only one time spine per granularity
         time_spines_by_granularity: Dict[TimeGranularity, List[TimeSpine]] = {}
         granularities_with_multiple_time_spines: Set[TimeGranularity] = set()
-        for time_spine in semantic_manifest.project_configuration.time_spines:
+        for time_spine in time_spines:
             granularity = time_spine.primary_column.time_granularity
             if granularity in time_spines_by_granularity:
                 time_spines_by_granularity[granularity].append(time_spine)

tests/validations/test_time_spines.py

@@ -127,6 +127,42 @@ def test_no_warning_for_legacy_time_spine() -> None:  # noqa: D
     assert len(issues.warnings) == 0
 
 
+def test_no_time_spine_config() -> None:  # noqa: D
+    validator = SemanticManifestValidator[PydanticSemanticManifest]()
+    semantic_manifest = PydanticSemanticManifest(
+        semantic_models=[
+            semantic_model_with_guaranteed_meta(
+                name="sum_measure",
+                dimensions=[
+                    PydanticDimension(
+                        name="dim",
+                        type=DimensionType.TIME,
+                        description="",
+                        metadata=None,
+                        type_params=PydanticDimensionTypeParams(time_granularity=TimeGranularity.SECOND),
+                    )
+                ],
+                entities=[PydanticEntity(name="entity", type=EntityType.PRIMARY)],
+            ),
+        ],
+        metrics=[
+            PydanticMetric(
+                name="metric",
+                description=None,
+                type=MetricType.SIMPLE,
+                type_params=PydanticMetricTypeParams(measure=PydanticMetricInputMeasure(name="sum_measure")),
+            ),
+        ],
+        project_configuration=PydanticProjectConfiguration(
+            time_spine_table_configurations=[],
+            time_spines=[],
+        ),
+    )
+    issues = validator.validate_semantic_manifest(semantic_manifest)
+    assert not issues.has_blocking_issues
+    assert len(issues.warnings) == 0
+
+
 def test_duplicate_time_spine_granularity() -> None:  # noqa: D
     validator = SemanticManifestValidator[PydanticSemanticManifest]()
     semantic_manifest = PydanticSemanticManifest(