RFR: 8260342: FXMLLoader fails to load a sub layout using fx:include with the resources attribute

[Guillaume Tâche]

This fixes ResourceBundle loading by calling `ResourceBundle.getBundle(value, Locale.getDefault())` when the loader resources are null or their classloader is null.    
Apparently the original author of the bug report said they would submit a pull request, but it seems like it never happened.

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Commit messages:
 - 8260342: FXMLLoader fails to load a sub layout using fx:include with the resources attribute

Changes: https://git.openjdk.org/jfx/pull/1158/files
 Webrev: https://webrevs.openjdk.org/?repo=jfx&pr=1158&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8260342
  Stats: 208 lines in 8 files changed: 205 ins; 0 del; 3 mod
  Patch: https://git.openjdk.org/jfx/pull/1158.diff
  Fetch: git fetch https://git.openjdk.org/jfx.git pull/1158/head:pull/1158

PR: https://git.openjdk.org/jfx/pull/1158