Less generation when no messages, enums, or extensions.

There's no need to generate any `import` directives or the version check
when the file is completely empty, and with different access levels on
imports that can get flagged with a newer compilers.

This is a common case when a .proto file contains a service.

Regenerate to get the one file update.

Reference/upstream/google/protobuf/unittest_empty.pb.swift

@@ -24,15 +24,4 @@
 // This file intentionally left blank.  (At one point this wouldn't compile
 // correctly.)
 
-import Foundation
-import SwiftProtobuf
-
-// If the compiler emits an error on this type, it is because this file
-// was generated by a version of the `protoc` Swift plug-in that is
-// incompatible with the version of SwiftProtobuf to which you are linking.
-// Please ensure that you are building against the same version of the API
-// that was used to generate this file.
-fileprivate struct _GeneratedWithProtocGenSwiftVersion: SwiftProtobuf.ProtobufAPIVersionCheck {
-  struct _2: SwiftProtobuf.ProtobufAPIVersion_2 {}
-  typealias Version = _2
-}
+// This file contained no messages, enums, or extensions.

Sources/protoc-gen-swift/FileGenerator.swift

@@ -85,18 +85,25 @@ class FileGenerator {
         } else if let location = fileDescriptor.sourceCodeInfoLocation(path: syntaxPath) {
             commentLocation = location
         }
-        var comments = String()
         if let commentLocation = commentLocation {
-          comments = commentLocation.asSourceComment(commentPrefix: "///",
-                                                     leadingDetachedPrefix: "//")
-          // If the was a leading or tailing comment it won't have a blank
-          // line, after it, so ensure there is one.
-          if !comments.isEmpty && !comments.hasSuffix("\n\n") {
-            comments.append("\n")
+          let comments = commentLocation.asSourceComment(commentPrefix: "///",
+                                                         leadingDetachedPrefix: "//")
+          if !comments.isEmpty {
+              // If the was a leading or tailing comment it won't have a blank
+              // line, after it, so ensure there is one.
+              p.print(comments, newlines: !comments.hasSuffix("\n\n"))
           }
         }
 
-        p.print("\(comments)\(generatorOptions.importDirective.snippet) Foundation")
+        // If there is nothing to generate, then just record that and be done (usually means
+        // there just was one or more services).
+        let generateEmpty = fileDescriptor.enums.isEmpty && fileDescriptor.messages.isEmpty && fileDescriptor.extensions.isEmpty
+        guard !generateEmpty else {
+            p.print("// This file contained no messages, enums, or extensions.")
+            return
+        }
+
+        p.print("\(generatorOptions.importDirective.snippet) Foundation")
 
         if fileDescriptor.isBundledProto {
             p.print("// 'import \(namer.swiftProtobufModuleName)' suppressed, this proto file is meant to be bundled in the runtime.")