Improve error message for empty array

When `jest-codemods` was run, it replaced the error message `array.join`
by a comment for the error message. Since Jest does not support custom
error messages out-of-the-box, this will instead do an equality check
with an empty array, which will ensure that the received array is
printed.

extensions/ql-vscode/test/pure-tests/sarif-processing.test.ts

@@ -645,8 +645,7 @@ describe("SARIF processing", () => {
 
   function expectNoParsingError(result: { errors: string[] }) {
     const array = result.errors;
-    // array.join()
-    expect(array.length).toBe(0);
+    expect(array).toEqual([]);
   }
 
   function buildValidSarifLog(): sarif.Log {