[clang-format] Add a testcase for empty macro

Author: amane-ame

clang/test/Format/empty-macro.cpp

@@ -0,0 +1,5 @@
+// RUN: grep -Ev "// *[A-Z-]+:" %s | clang-format -style="{Macros: [A(x)=x]}" \
+// RUN:   | FileCheck -strict-whitespace %s
+
+// CHECK: A()
+A()