Empty CFStringRefthrows Exception if stringValue is called (macOS)

[dyorgio]

1. Version of JNA and related jars
   5.6.0 platform jar.
2. Version and vendor of the java virtual machine
   Azul Zulu (zulu8.46.0.19-ca-fx-jdk8.0.252-macosx_x64)
3. Operating system
   macOS Big Sur (11.0.1)
4. System architecture (CPU type, bitness of the JVM)
   x64, x64
5. Complete description of the problem
   Method CFStringRef.stringValue throws java.lang.IllegalArgumentException: Allocation size must be greater than zero if used in a empty string (char[0)) instance.
6. Steps to reproduce
   Just call:

String var = CoreFoundation.CFStringRef.createCFString("").stringValue();

[dbwiddis]

Thanks for the report and the fix. Surprised I didn't run into that edge case before!