Stencil Upload allows upload of filedescriptorset without imports for the first time.

[sravankorumilli]

Describe the bug
Stencil Upload allows upload of filedescriptorset for the first time

To Reproduce
Steps to reproduce the behavior:

1. Create a FileDescriptorFileSet without without --include_imports
2. Upload the same it succeeds for the first time.

Expected behavior
Should fail