app: also check for an empty name.

I'm looking at #14014. It's probably not the problem either, especially
as gimp_parasite_new() would return a NULL parasite anyway with an empty
name.

But better be thorough and output a sensible error as soon as possible
when loading a XCF.

app/xcf/xcf-load.c

@@ -4782,7 +4782,7 @@ xcf_load_parasite (XcfInfo *info)
       return NULL;
     }
 
-  if (!name)
+  if (! name || strlen (name) == 0)
     {
       g_printerr ("Parasite has no name! Possibly corrupt XCF file.\n");
       return NULL;