Avoid exception when getting zero-length xattr on mac

[jrobhoward]

On MacOS, reading a zero-sized xattr will result in an IllegalArgumentException:

Exception in thread "main" java.lang.IllegalArgumentException: Allocation size must be greater than zero
	at com.sun.jna.Memory.<init>(Memory.java:111)
	at com.sun.jna.platform.mac.XAttrUtil.getXAttr(XAttrUtil.java:61)

The code change associated with this pull request returns an empty string before attempting to allocate Memory(0).

• Allowing the caller to distinguish between (an xattr that does not exist, return value=null) and (an empty xattr, return value="").

This only appears to be a problem on MacOS' implementation of getXAttr(). Linux will return an empty string.

• The Linux version does have getXAttrAsMemory() methods that will produce a similar problem, but I'm not sure what would be the most appropriate: (IllegalArgumentException, IOException, null return value) --so I didn't make any changes.

[matthiasblaesing]

Thank you - the change looks reasonable. Could you please also add an entry to the Bugfixes section of CHANGES.md? Thank you.

For the other methods: I would not use exceptions - it is not exceptional, that a value is not set/set to the empty string. So if a method returns a Memory object and a length of 0 is indicated, I would return null for the Memory object. For a byte[] there is a valid value representing the "empty" byte array.

[jrobhoward]

I added the changelog entry, and updated the Linux XAttrUtil implementation so it returns null instead of throwing exceptions when attempting to create zero-length Memory objects. I did not alter getXAttrBytes(), since it should already be returning a valid zero-byte array.

[matthiasblaesing]

Merged via: bc22845

Your changes are completely included - I added unittests to verify the behavior and moved the changelog entry to the correct place.

I have to apologize, the commit only lists me as author - I did multiple rebases and somewhere along the way, your authorship got lost. I ensured though, that you are listed as author in the CHANGES.md.