Add quotes to sourcepath

If Srcs contains only srcjar, sourcepath is null and an error occurs. So
if sourcepath is null, we will use the "-sourcepath "" ".

Test: m -j
Bug: 112397488
Change-Id: I03ac0074fc041203fa1b427d4b4a418af44e85e2

java/droiddoc.go

@@ -1307,8 +1307,7 @@ func (d *Droidstubs) initBuilderFlags(ctx android.ModuleContext, implicits *andr
 	}
 	flags.classpathArgs = deps.classpath.FormJavaClassPath("-classpath")
 
-	flags.sourcepathArgs = "-sourcepath " + strings.Join(d.Javadoc.sourcepaths.Strings(), ":")
-
+	flags.sourcepathArgs = "-sourcepath \"" + strings.Join(d.Javadoc.sourcepaths.Strings(), ":") + "\""
 	return flags, nil
 }