Ignore empty directories for sources

Empty directories should not matter for source files, so let's ignore
them for up-to-date checks as well.

Author: wolfs

src/main/groovy/com/google/protobuf/gradle/GenerateProtoTask.groovy

@@ -29,6 +29,8 @@
  */
 package com.google.protobuf.gradle
 
+import org.gradle.api.tasks.IgnoreEmptyDirectories
+
 import static java.nio.charset.StandardCharsets.US_ASCII
 
 import com.google.common.base.Preconditions
@@ -302,8 +304,9 @@ public abstract class GenerateProtoTask extends DefaultTask {
   }
 
   @SkipWhenEmpty
-  @InputFiles
   @PathSensitive(PathSensitivity.RELATIVE)
+  @IgnoreEmptyDirectories
+  @InputFiles
   FileCollection getSourceFiles() {
     return sourceFiles
   }