Merge pull request swiftlang#21940 from compnerd/null-is-empty

test: define expansion macro always

Author: compnerd

test/lit.cfg

@@ -1275,6 +1275,8 @@ config.substitutions.append(('%target-clang', config.target_clang))
 config.substitutions.append(('%target-ld', config.target_ld))
 if hasattr(config, 'target_cc_options'):
     config.substitutions.append(('%target-cc-options', config.target_cc_options))
+else:
+    config.substitutions.append(('%target-cc-options', ''))
 
 config.substitutions.append(
     (r'%hardlink-or-copy\(from: *(.*), *to: *(.*)\)',