Add test for #if , (leading comma syntax error)

Author: swote-git

clang/test/Preprocessor/cxx_oper_comma.cpp

@@ -30,4 +30,9 @@
 // Test 5: Comma in #elif
 #if 0
 #elif (1, 2) // expected-error {{comma operator in operand of #if}}
+#endif
+
+// Test 6: Leading comma (syntax error)
+// expected-error@+1 {{invalid token at start of a preprocessor expression}}
+#if ,
 #endif