altair: make sure parse_vendor_pco_info() always returns an error

The parse_vendor_pco_info() method was returning NULL without error if
the pco info string was empty.

Under this situation, the code would have tried to add a NULL MMPco
into the pco_list list, which is not desired.

Avoid this, by making sure a NULL return always sets an error.

plugins/altair/mm-broadband-modem-altair-lte.c

@@ -1105,9 +1105,13 @@ altair_pco_info_changed (MMPortSerialAt *port,
 
     pco_info = g_match_info_fetch (match_info, 0);
 
+    /* ignore if empty */
+    if (!pco_info || !pco_info[0])
+        return;
+
     mm_obj_dbg (self, "parsing vendor PCO info: %s", pco_info);
     pco = mm_altair_parse_vendor_pco_info (pco_info, &error);
-    if (error) {
+    if (!pco) {
         mm_obj_warn (self, "error parsing vendor PCO info: %s", error->message);
         return;
     }

plugins/altair/mm-modem-helpers-altair-lte.c

@@ -140,9 +140,11 @@ mm_altair_parse_vendor_pco_info (const gchar *pco_info, GError **error)
     MMPco *pco = NULL;
     gint num_matches;
 
-    if (!pco_info[0])
-        /* No APNs configured, all done */
+    if (!pco_info || !pco_info[0]) {
+        g_set_error (error, MM_CORE_ERROR, MM_CORE_ERROR_INVALID_ARGS,
+                     "No PCO info given");
         return NULL;
+    }
 
     /* Expected %PCOINFO response:
      *