ublox: always send user/pass strings, even if no authentication requested

The TOBY-L2 allowed to skup the user/pass string fields when no
authentication was requested, but according to the AT command
reference, all the remaining u-blox modules do require these two
fields given always (e.g. just as empty strings).

As per this sequence in a TOBY-L4:

 (ttyACM2): --> 'AT+UAUTHREQ=?<CR>'
 (ttyACM2): <-- '<CR><LF>+UAUTHREQ: (1-4),(0-2),,<CR><LF><CR><LF>OK<CR><LF>'
 (ttyACM2): --> 'AT+UAUTHREQ=1,0<CR>'
 (ttyACM2): <-- '<CR><LF>+CME ERROR: 4<CR><LF>'

It should have been: AT+UAUTHREQ=1,0,"",""

plugins/ublox/mm-broadband-bearer-ublox.c

@@ -463,7 +463,7 @@ out:
         g_free (quoted_user);
         g_free (quoted_password);
     } else
-        cmd = g_strdup_printf ("+UAUTHREQ=%u,0", ctx->cid);
+        cmd = g_strdup_printf ("+UAUTHREQ=%u,0,\"\",\"\"", ctx->cid);
 
     mm_dbg ("setting up authentication preferences in PDP context #%u...", ctx->cid);
     mm_base_modem_at_command (MM_BASE_MODEM (ctx->modem),