Made builtins cache valid only if it has a valid reference to the builtins file (PY-11665)

A temporary builtins cache with no builtins file has been considered
valid before and thus has been never invalidated.

python/src/com/jetbrains/python/psi/impl/PyBuiltinCache.java

@@ -172,7 +172,7 @@ public class PyBuiltinCache {
   }
 
   public boolean isValid() {
-    return myBuiltinsFile == null || myBuiltinsFile.isValid();
+    return myBuiltinsFile != null && myBuiltinsFile.isValid();
   }
 
   /**