Explicitly check for null values in C# jsonization

We explicitly check that both optional and required properties can not
be set to ``null``. The optional properties should be explicitly absent
from the object by specification, and thus ``null`` is not an acceptable
value for optional properties.

aas_core_codegen/csharp/jsonization/_generate.py

@@ -85,7 +85,7 @@ def _generate_from_method_for_interface(
 if (obj == null)
 {{
 {I}error = new Reporting.Error(
-{II}"Expected Nodes.JsonObject, but got {{node.GetType()}}");
+{II}$"Expected Nodes.JsonObject, but got {{node.GetType()}}");
 {I}return null;
 }}"""
         ),
@@ -336,6 +336,42 @@ def _generate_deserialize_constructor_argument(
     else:
         assert_never(arg.type_annotation)
 
+    # NOTE (mristin):
+    # We need to add a prologue to the parsing body to explicitly check for null
+    # values as the null values are not allowed for optional properties by
+    # specification.
+    if isinstance(arg.type_annotation, intermediate.OptionalTypeAnnotation):
+        parse_block = Stripped(
+            f"""\
+if (keyValue.Value == null)
+{{
+{I}error = new Reporting.Error(
+{II}"Expected optional property to be absent, " +
+{II}"but got null instead");
+{I}error.PrependSegment(
+{II}new Reporting.NameSegment(
+{III}{json_literal}));
+{I}return null;
+}}
+
+{parse_block}"""
+        )
+    else:
+        parse_block = Stripped(
+            f"""\
+if (keyValue.Value == null)
+{{
+{I}error = new Reporting.Error(
+{II}"Unexpected null for a required property");
+{I}error.PrependSegment(
+{II}new Reporting.NameSegment(
+{III}{json_literal}));
+{I}return null;
+}}
+
+{parse_block}"""
+        )
+
     return parse_block, None
 
 
@@ -398,21 +434,11 @@ def _generate_from_method_for_class(
             assert case_body is not None
             json_name = naming.json_property(arg.name)
 
-            # NOTE (mristin, 2022-07-23):
-            # We put ``if (keyValue.Value != null)`` here instead of the outer loop
-            # since we want to detect the unexpected additional properties even
-            # though their value can be set to null.
-
             cases.append(
                 Stripped(
                     f"""\
 case {csharp_common.string_literal(json_name)}:
 {{
-{I}if (keyValue.Value == null)
-{I}{{
-{II}continue;
-{I}}}
-
 {I}{indent_but_first_line(case_body, I)}
 {I}break;
 }}"""