Solve issue #492.
The nullity check is not enough anymore since we've tried to avoid null
values in PenStroke fields. We must check that the dashArray is not
empty before trying to find double values in it. If we do not, we'll
obtain an NumberFormatException.
Fix a problem with the apply button in the advanced editor.
It replaced the list of available styles, in the TOC, with a new list
containing only the style being edited currently.
Merge pull request #493 from agueganno/bug-hatches

    * Draw a HatchedFill on a polygon layer with an empty dash array (previously impossible)
    * Don't delete all styles except the one being edited when clicking Apply in the Advanced Style Editor.