fix(tests): fail if copyright message is not found

Missed the case when the copyright notice does not appear anywhere in
the file.

Signed-off-by: Pablo Barbáchano <pablob@amazon.com>

Author: pb8o

tests/integration_tests/style/test_licenses.py

@@ -57,6 +57,8 @@ def _validate_license(filename):
             if line.startswith(("// Copyright", "# Copyright")):
                 copyright_info = line
                 break
+            if line == "":
+                return False
 
         has_amazon_copyright = _has_amazon_copyright(
             copyright_info