spirv-fuzz: Quit fuzzer pass when no types are available (#4409)

The fuzzer pass that adds global variables requires some basic
types. This change makes the fuzzer pass exit gracefully when none are

Fixes #4408.
diff --git a/source/fuzz/fuzzer_pass_add_global_variables.cpp b/source/fuzz/fuzzer_pass_add_global_variables.cpp
index 0679741..be78ee5 100644
--- a/source/fuzz/fuzzer_pass_add_global_variables.cpp
+++ b/source/fuzz/fuzzer_pass_add_global_variables.cpp
@@ -47,6 +47,10 @@
   // These are the basic types that are available to this fuzzer pass.
   auto& basic_types = basic_type_ids_and_pointers.first;
+  if (basic_types.empty()) {
+    // There are no basic types, so there is nothing this fuzzer pass can do.
+    return;
+  }
   // These are the pointers to those basic types that are *initially* available
   // to the fuzzer pass.  The fuzzer pass might add pointer types in cases where