[fuzzers] Substitute all override variables

The transform was only being run if there was at least one override
without an initializer. We actually need to run the transform if there
are any overrides at all, even if they all have initializers.

Bug: chromium:1447418
Change-Id: Ia300ea4326b675c1fd84dd7709112fb69128963d
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/134203
Reviewed-by: Dan Sinclair <dsinclair@chromium.org>
Kokoro: Kokoro <noreply+kokoro@google.com>
Commit-Queue: James Price <jrprice@google.com>

src/tint/fuzzers/tint_common_fuzzer.cc

@@ -248,7 +248,7 @@ int CommonFuzzer::Run(const uint8_t* data, size_t size) {
             cfg.map.insert({override_id, 0.0});
         }
 
-        if (!cfg.map.empty()) {
+        if (!default_values.empty()) {
             transform::DataMap override_data;
             override_data.Add<ast::transform::SubstituteOverride::Config>(cfg);