Fix exception for unlinked files

[ZhouSky]

Fixes #5815 Exception when importing certain unlinked local files

1. For some certain files, they have metadata, but cannot find start or end tag, so cause exception. I added statements to check if can find start or end tag, and if the position of start tag before end tag in metadata.
2. Original private List<Path> getFileListFromNode(CheckBoxTreeItem<FileNodeWrapper> node) method just can find children nodes, so causes that files under subfolder cannot be found, then cannot be imported. I modified this method to iterate find all selected leaf nodes.

• Change in CHANGELOG.md described (if applicable)
• Tests created for changes (if applicable)
• Manually tested changed features in running JabRef (always required)
• Screenshots added in PR description (for UI changes)
• Checked documentation: Is the information available and up to date? If not created an issue at https://github.com/JabRef/user-documentation/issues or, even better, submitted a pull request to the documentation repository.

[Siedlerchr]

It returns an empty list, not an optional. Please adjust thar javadoc comment
Otherwise looks good

[ZhouSky]

Got it

[ZhouSky]

Thanks for the reminder

[Siedlerchr]

Just another wish, it would be really cool if you could a test for the fix in the XMPUtilReader as well, see
XmpUtilReaderTest

[ZhouSky]

Ok, i'll try.

[ZhouSky]

Done

[Siedlerchr]

You don't need reflection, the method getMetadata is implicitly caleld, you you can simple do this two lines:

   List<BibEntry> entries = XmpUtilReader.readXmp(Path.of(XmpUtilShared.class.getResource("no_description_metadata.pdf").toURI()), xmpPreferences);
        assertEquals(Collections.emptyList(), entries);

[ZhouSky]

Got it. Thanks.

[ZhouSky]

Done.

[Siedlerchr]

Thanks for the quick follow up!