JabRef error if keyword group has no name

From jabref created by [alfureu](https://github.com/alfureu): JabRef/jabref#9776

**Update**: Read on at https://github.com/JabRef/jabref/issues/9776#issuecomment-1555866023

### JabRef version

5.9 (latest release)

### Operating system

macOS

### Details on version and operating system

macOS 13.3.1

### Checked with the latest development build

- [X] I made a backup of my libraries before testing the latest development version.
- [X] I have tested the latest development version and the problem persists

### Steps to reproduce the behaviour

1. import a biblatex library created on linux machine and with the special field `priority`
2. error opening (does not open)

```
JabRef 5.9--2023-01-08--76253f1a7
Mac OS X 13.3.1 aarch64
Java 19.0.1
JavaFX 19+11
```

### Appendix

...

Log File

```
java.lang.NumberFormatException: For input string: "priority"
at java.base/java.lang.NumberFormatException.forInputString(Unknown Source)
at java.base/java.lang.Integer.parseInt(Unknown Source)
at java.base/java.lang.Integer.parseInt(Unknown Source)
at org.jabref@5.9.60000/org.jabref.logic.importer.util.GroupsParser.keywordGroupFromString(Unknown Source)
at org.jabref@5.9.60000/org.jabref.logic.importer.util.GroupsParser.fromString(Unknown Source)
at org.jabref@5.9.60000/org.jabref.logic.importer.util.GroupsParser.importGroups(Unknown Source)
at org.jabref@5.9.60000/org.jabref.logic.importer.util.MetaDataParser.parse(Unknown Source)
at org.jabref@5.9.60000/org.jabref.logic.importer.util.MetaDataParser.parse(Unknown Source)
at org.jabref@5.9.60000/org.jabref.logic.importer.fileformat.BibtexParser.parseFileContent(Unknown Source)
at org.jabref@5.9.60000/org.jabref.logic.importer.fileformat.BibtexParser.parse(Unknown Source)
at org.jabref@5.9.60000/org.jabref.logic.importer.fileformat.BibtexImporter.importDatabase(Unknown Source)
at org.jabref@5.9.60000/org.jabref.logic.importer.fileformat.BibtexImporter.importDatabase(Unknown Source)
at org.jabref@5.9.60000/org.jabref.logic.importer.OpenDatabase.loadDatabase(Unknown Source)
at org.jabref@5.9.60000/org.jabref.gui.importer.actions.OpenDatabaseAction.loadDatabase(Unknown Source)
at org.jabref@5.9.60000/org.jabref.gui.importer.actions.OpenDatabaseAction.lambda$openTheFile$1(Unknown Source)
at org.jabref@5.9.60000/org.jabref.gui.util.BackgroundTask$1.call(Unknown Source)
at org.jabref@5.9.60000/org.jabref.gui.util.DefaultTaskExecutor$1.call(Unknown Source)
at org.jabref.merged.module@5.9.60000/javafx.concurrent.Task$TaskCallable.call(Unknown Source)
at java.base/java.util.concurrent.FutureTask.run(Unknown Source)
at java.base/java.util.concurrent.Executors$RunnableAdapter.call(Unknown Source)
at java.base/java.util.concurrent.FutureTask.run(Unknown Source)
at java.base/java.util.concurrent.ThreadPoolExecutor.runWorker(Unknown Source)
at java.base/java.util.concurrent.ThreadPoolExecutor$Worker.run(Unknown Source)
at java.base/java.lang.Thread.run(Unknown Source)

```

Read the linked JabRef issue update first, then inspect GroupsParser.keywordGroupFromString and the importer call path shown in the stack trace. Reproduce the failure by opening a BibLaTeX library containing the priority field; done means the library opens without the NumberFormatException.

Written by the indexing model from the issue text.