[Support][NFC] Add test documenting that empty Regex pattern matches nothing. (llvm#83849)

I was wondering about this when I recently used `Regex`, and I thought
it would
be nice to have a test documenting this behavior.

Author: martinboehme

llvm/unittests/Support/RegexTest.cpp

@@ -60,6 +60,14 @@ TEST_F(RegexTest, Basics) {
   EXPECT_TRUE(r5.match(String));
 }
 
+TEST_F(RegexTest, EmptyPattern) {
+  // The empty pattern doesn't match anything -- not even the empty string.
+  // (This is different from some other regex implementations.)
+  Regex r("");
+  EXPECT_FALSE(r.match("123"));
+  EXPECT_FALSE(r.match(""));
+}
+
 TEST_F(RegexTest, Backreferences) {
   Regex r1("([a-z]+)_\\1");
   SmallVector<StringRef, 4> Matches;