runtime.c: Set `XDG_DATA_DIRS` if it is empty to begin with

I have a GTK# mono APP, in ubuntu and debian runs without problems.
However in manjaro 17.0.2 I have problems with GTK theme, get errors loading png icons.

```
** (SerialTerminal:1190): WARNING **: Pixbuf theme: Cannot load pixmap file /usr/share/themes/Vertex-Maia-Dark/gtk-2.0/Entry/entry-border-fill.png: No se pudo reconocer el formato de imagen del archivo «/usr/share/themes/Vertex-Maia-Dark/gtk-2.0/Entry/entry-border-fill.png»

(SerialTerminal:1190): GdkPixbuf-CRITICAL **: gdk_pixbuf_get_width: assertion 'GDK_IS_PIXBUF (pixbuf)' failed

(SerialTerminal:1190): GdkPixbuf-CRITICAL **: gdk_pixbuf_get_height: assertion 'GDK_IS_PIXBUF (pixbuf)' failed

** (SerialTerminal:1190): WARNING **: Invalid borders specified for theme pixmap:
/usr/share/themes/Vertex-Maia-Dark/gtk-2.0/Entry/entry-border-fill.png,
borders don't fit within the image
```
runing the application directly whitout create appimage load the icons correctly, so the problem is the XDG_DATA_DIRS, in manajro this variable is not set so when AppRun loader set the XDG_DATA_DIRS put only one path in the variable, and the icons can't be loaded.
adding
`export XDG_DATA_DIRS=$(printenv XDG_DATA_DIRS):/usr/local/share:/usr/share`
to my app bash script solves de problem.

However I thing that the AppRun loader must set correctly de XDG_DATA_DIRS.

https://specifications.freedesktop.org/basedir-spec/basedir-spec-latest.html