Add test for linting empty source

flake8_annotations/checker.py

@@ -43,7 +43,7 @@ def __init__(self, tree: t.Optional[ast.Module], lines: t.List[str]):
         # Type ignores are provided by ast at the module level & we'll need them later when deciding
         # whether or not to emit errors for a given function
         self._type_ignore_lineno = {ti.lineno for ti in self.tree.type_ignores}
-        self._has_mypy_ignore_errors = "# mypy: ignore-errors" in lines[0]
+        self._has_mypy_ignore_errors = "# mypy: ignore-errors" in lines[0] if lines else False
 
         # Set by flake8's config parser
         self.suppress_none_returning: bool
@@ -119,7 +119,9 @@ def run(self) -> t.Generator[FORMATTED_ERROR, None, None]:
                 if function.lineno in self._type_ignore_lineno:
                     # function-level ignore
                     continue
-                elif (1 in self._type_ignore_lineno) or (self._has_mypy_ignore_errors):
+                elif (1 in self._type_ignore_lineno) or (
+                    self._has_mypy_ignore_errors
+                ):  # pragma: no branch
                     # module-level ignore
                     # lineno from ast is 1-indexed
                     continue

testing/test_empty_src.py

@@ -0,0 +1,6 @@
+from testing.helpers import check_source
+
+
+def test_empty_source() -> None:
+    errs = check_source("")
+    assert len(list(errs)) == 0