Add empty expression test case

Author: AA-Turner

flit_core/flit_core/config.py

@@ -833,7 +833,7 @@ def normalise_compound_license_expr(s: str) -> str:
     Spec: https://spdx.github.io/spdx-spec/v2.2.2/SPDX-license-expressions/
     """
     invalid_msg = "'{s}' is not a valid SPDX license expression: {reason}"
-    if s == '':
+    if not s or s.isspace():
         raise ConfigError(f"The SPDX license expression must not be empty")
 
     stack = 0

flit_core/tests_core/test_config.py

@@ -327,6 +327,24 @@ def test_license_expr_error(invalid_expr: str):
         config.read_pep621_metadata(proj, samples_dir / 'pep621' / 'pyproject.toml')
 
 
+@pytest.mark.parametrize('invalid_expr', [
+    "",
+    " ",
+    "\t",
+    "\r",
+    "\n",
+    "\f",
+    " \t \n \r \f ",
+])
+def test_license_expr_error_empty(invalid_expr: str):
+    proj = {
+        'name': 'module1', 'version': '1.0', 'description': 'x',
+        'license': invalid_expr,
+    }
+    with pytest.raises(config.ConfigError, match="must not be empty"):
+        config.read_pep621_metadata(proj, samples_dir / 'pep621' / 'pyproject.toml')
+
+
 @pytest.mark.parametrize('invalid_expr', [
     "mit or mit",
     "or",