reject empty xresources (#340)

currently, in case of an empty xresources we would fail to init:

	nsxiv: Error allocating color ''

instead, just reject empty value and use the fallback.

Closes: https://codeberg.org/nsxiv/nsxiv/issues/339

Reviewed-on: https://codeberg.org/nsxiv/nsxiv/pulls/340
Reviewed-by: Berke Kocaoğlu <berke.kocaoglu@metu.edu.tr>
Co-authored-by: NRK <nrk@disroot.org>
Co-committed-by: NRK <nrk@disroot.org>

window.c

@@ -95,7 +95,7 @@ static const char* win_res(XrmDatabase db, const char *name, const char *def)
 
 	if (db != NULL &&
 	    XrmGetResource(db, name, name, &type, &ret) &&
-	    STREQ(type, "String"))
+	    STREQ(type, "String") && *ret.addr != '\0')
 	{
 		return ret.addr;
 	} else {