[mksnapshot] Default-init embedded variant to "Default"

This was always the intended behavior: when --embedded-variant is not
passed, we should assume "Default". What actually happened in this
case was that we'd initialize to nullptr. This CL adds the missing
nullptr check.

Bug: v8:6666
Change-Id: I87247c6c6cfec20da86d2714732a2f6044bb9b77
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1526193
Commit-Queue: Ulan Degenbaev <ulan@chromium.org>
Auto-Submit: Jakob Gruber <jgruber@chromium.org>
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#60278}

src/snapshot/mksnapshot.cc

@@ -275,7 +275,9 @@ int main(int argc, char** argv) {
 
     i::EmbeddedFileWriter embedded_writer;
     embedded_writer.SetEmbeddedFile(i::FLAG_embedded_src);
-    embedded_writer.SetEmbeddedVariant(i::FLAG_embedded_variant);
+    if (i::FLAG_embedded_variant != nullptr) {
+      embedded_writer.SetEmbeddedVariant(i::FLAG_embedded_variant);
+    }
 
     std::unique_ptr<char> embed_script(
         GetExtraCode(argc >= 2 ? argv[1] : nullptr, "embedding"));