windows: fix --symlink_prefix=/ throwing exception

Fixes #4421

RELNOTES: none
PiperOrigin-RevId: 181742216

Author: laszlocsomor

src/main/java/com/google/devtools/build/lib/buildtool/OutputDirectoryLinksUtils.java

@@ -154,6 +154,10 @@ static void createOutputDirectoryLinks(
    */
   public static PathFragment getPrettyPath(Path file, String workspaceName,
       Path workspaceDirectory, String symlinkPrefix, String productName) {
+    if (NO_CREATE_SYMLINKS_PREFIX.equals(symlinkPrefix)) {
+      return file.asFragment();
+    }
+
     for (String link : LINKS) {
       PathFragment result = relativize(file, workspaceDirectory, symlinkPrefix + link);
       if (result != null) {