Fix implicit std::string_view(nullptr) (#28589)

This fixes failure to compile on (pre-release) GCC 12 due to use of
explicitly deleted function:

test/core/security/evaluate_args_test.cc

@@ -32,9 +32,9 @@ class EvaluateArgsTest : public ::testing::Test {
 
 TEST_F(EvaluateArgsTest, EmptyMetadata) {
   EvaluateArgs args = util_.MakeEvaluateArgs();
-  EXPECT_EQ(args.GetPath(), nullptr);
-  EXPECT_EQ(args.GetMethod(), nullptr);
-  EXPECT_EQ(args.GetAuthority(), nullptr);
+  EXPECT_THAT(args.GetPath(), ::testing::IsEmpty());
+  EXPECT_THAT(args.GetMethod(), ::testing::IsEmpty());
+  EXPECT_THAT(args.GetAuthority(), ::testing::IsEmpty());
   EXPECT_EQ(args.GetHeaderValue("some_key", nullptr), absl::nullopt);
 }