Do not emit warning when there are no proto files

[alexander-yevsyukov]

When building model-tools/model-check I got the below warning:

> Task :model-check:annotateProto
Missing descriptor set file `/Users/sanders/Projects/Spine/model-tools/model-check/build/descriptors/main/io.spine.tools_model-check_2.0.0-SNAPSHOT.111.desc` produced for the source set `main` which has `proto` extension.
Please enable descriptor set generation.
See: https://github.com/google/protobuf-gradle-plugin/blob/master/README.md#generate-descriptor-set-files

The module does use McJava and has proto files used for Tests. But it does not have proto files in the main source set. As such, the module does have the proto extension, but the build does not produce descriptor set files because there are no proto files.

McData should see if there are proto files in the source set and emit the warning only then. The current behaviour is misleading.