Fix getting env variables on windows (#15518)

Fixes https://github.com/protocolbuffers/protobuf/issues/15436

Closes #15518

COPYBARA_INTEGRATE_REVIEW=https://github.com/protocolbuffers/protobuf/pull/15518 from isuruf:win_env 1b1f2cc31c
PiperOrigin-RevId: 600469988

src/google/protobuf/compiler/mock_code_generator.cc

@@ -72,7 +72,13 @@ static constexpr absl::string_view kSecondInsertionPoint =
     "  # @@protoc_insertion_point(second_mock_insertion_point) is here\n";
 
 MockCodeGenerator::MockCodeGenerator(absl::string_view name) : name_(name) {
-  absl::string_view key = getenv("TEST_CASE");
+  const char* c_key = getenv("TEST_CASE");
+  if (c_key == NULL) {
+    // In Windows, setting 'TEST_CASE=' is equivalent to unsetting
+    // and therefore c_key can be NULL
+    c_key = "";
+  }
+  absl::string_view key(c_key);
   if (key == "no_editions") {
     suppressed_features_ |= CodeGenerator::FEATURE_SUPPORTS_EDITIONS;
   } else if (key == "invalid_features") {