Make empty generated source files descriptive

Motivation:

Sometimes protobuf definition files contain no gRPC services and result
in an empty file. This is intentional but could be confusing.

Modifications:

Empty files now contain a comment indicating they are intentional.
```
// This file contained no services.
```

Result:

More descriptive empty source files.

Author: rnro

Sources/GRPCCodeGen/SourceGenerator.swift

@@ -87,13 +87,21 @@ public struct SourceGenerator: Sendable {
     let translator = IDLToStructuredSwiftTranslator()
     let textRenderer = TextBasedRenderer(indentation: self.config.indentation)
 
-    let structuredSwiftRepresentation = try translator.translate(
+    var structuredSwiftRepresentation = try translator.translate(
       codeGenerationRequest: request,
       accessLevel: self.config.accessLevel,
       accessLevelOnImports: self.config.accessLevelOnImports,
       client: self.config.client,
       server: self.config.server
     )
+
+    if structuredSwiftRepresentation.file.contents.codeBlocks.isEmpty {
+      structuredSwiftRepresentation.file.contents.imports = []
+      structuredSwiftRepresentation.file.contents.codeBlocks.append(
+        CodeBlock(comment: .inline("This file contained no services."))
+      )
+    }
+
     let sourceFile = try textRenderer.render(structured: structuredSwiftRepresentation)
 
     return sourceFile