Generate code file for protos with no definitions

Motivation

To prepare for use in a SwiftPM build plugin which requires
deterministic output files and to match the behavior of
`proto-gen-swift` we should generate a source file even if no
definitions are found.

Modifications:

We no longer return early in the case of no definitions being found.

Result:

We will generate a Swift source file even if no definitions are found.

Sources/protoc-gen-grpc-swift/GenerateGRPC.swift

@@ -63,10 +63,6 @@ final class GenerateGRPC: CodeGenerator {
         )
       }
 
-      if descriptor.services.isEmpty {
-        continue
-      }
-
       try self.generateV2Stubs(descriptor, options: options, outputs: outputs)
     }
   }