Previously the ngfactory shim generator in ngtsc would always write two
imports in the factory file shims:
1) an import to @angular/core
2) an import to the base file
If the base file has no exports, import #2 would be empty. This turns out
to cause issues downstream.
This commit changes the generated shim so if there are no exports in the
base file, the generated shim is empty too.
PR Close#27470
345bdd3db0