In java/java.go, enhance the checking logic for Dependency interface implementation

Add nil checking for Dependency interface implementation

Bug: 121985695

Test: 1. m aidegen
      2. $ANDROID_HOST_OUT/bin/aidegen framework
      3. check import static android.Manifest.permission.BIND_VOICE_INTERACTION;
	 denpendency should be resolved in frameworks/base/services/core/java/com/android/server/am/ActivityManagerService.java

Change-Id: Ibac280ce3de2537f453c4ca7e8f79f55667f1fa0

java/java.go

@@ -1427,6 +1427,9 @@ func (j *Module) HeaderJars() android.Paths {
 }
 
 func (j *Module) ImplementationJars() android.Paths {
+	if j.implementationJarFile == nil {
+		return nil
+	}
 	return android.Paths{j.implementationJarFile}
 }
 
@@ -1809,6 +1812,9 @@ func (j *Import) HeaderJars() android.Paths {
 }
 
 func (j *Import) ImplementationJars() android.Paths {
+	if j.combinedClasspathFile == nil {
+		return nil
+	}
 	return android.Paths{j.combinedClasspathFile}
 }