fix(query): fix commands throwing error with empty sourceText.

closes #78

src/query/_shared/__tests__/__snapshots__/getTokenAtPosition.test.js.snap

@@ -1,5 +1,41 @@
 // Jest Snapshot v1, https://goo.gl/fbAQLP
 
+exports[`[Bug] should works for empty sourceText 1`] = `
+Object {
+  "end": 1,
+  "prevChar": "",
+  "start": 0,
+  "state": Object {
+    "indentLevel": 0,
+    "kind": "Document",
+    "level": 0,
+    "name": null,
+    "needsSeperator": false,
+    "prevState": Object {
+      "kind": null,
+      "level": 0,
+      "name": null,
+      "needsSeperator": false,
+      "prevState": null,
+      "rule": null,
+      "step": 0,
+      "type": null,
+    },
+    "rule": Array [
+      Object {
+        "isList": true,
+        "ofRule": "Definition",
+        "separator": undefined,
+      },
+    ],
+    "step": 0,
+    "type": null,
+  },
+  "string": " ",
+  "style": "ws",
+}
+`;
+
 exports[`[Bug] should works for start position 1`] = `
 Object {
   "end": 8,

src/query/_shared/__tests__/getTokenAtPosition.test.js

@@ -100,11 +100,16 @@ describe('should able to parse Relay.QL queries with interpolation', () => {
 
     expect(() => getTokenAtPosition(sourceText, position, relayQLParser)).not.toThrow();
   });
-
 });
 
 it('[Bug] should works for start position', () => {
   const sourceText = 'fragment';
   const token = getTokenAtPosition(sourceText, { line: 1, column: 1 }, 'QueryParser');
   expect(token).toMatchSnapshot();
 });
+
+it('[Bug] should works for empty sourceText', () => {
+  const sourceText = '';
+  const token = getTokenAtPosition(sourceText, { line: 1, column: 1 }, 'QueryParser');
+  expect(token).toMatchSnapshot();
+});

src/query/commands/__tests__/getHintsAtPosition.test.js

@@ -340,3 +340,10 @@ describe('[Bug] should work if position is first character', () => {
     ).toEqual([{ text: 'fragment' }]);
   });
 });
+
+test('[Bug] should work with empty sourceText', () => {
+  const sourceText = '';
+  expect(
+    getHintsAtPosition(schema, sourceText, { line: 1, column: 1 }, queryConfig),
+  ).toEqual(hints.DocumentLevel);
+});

src/shared/getTokenAtPosition.js

@@ -15,9 +15,10 @@ import toOffset from './toOffset';
 
 export default function getTokenAtPosition(
   parser: IParser,
-  sourceText: string,
+  _sourceText: string,
   position: Position,
 ): Token {
+  const sourceText = _sourceText ? _sourceText : ' ';
   const state = parser.startState();
   const offset = toOffset(sourceText, position);
   const stream = new MultilineCharacterStream(sourceText);