SLDParser / SLDTransformer fails to round-trip empty nodes.

Steps to reproduce:

Start with a Style containing a text symbolixer, with fontFamily = [""]

Using SLDTransformer, write the style to disk. Then read it in again using SLDParser.
Now, fontFamily = [null]
Write the new Style object, then read it in again. Now, fontFamily = [null]

This issue seems to be being introduced around line 2347 of SLDParser.