broadband-bearer-qmi: don't set APN if it isn't given

If the APN was empty (implying that the modem/network should choose
the default APN) this caused an assertion in libqmi.  Just don't
set an APN if we aren't given one.

src/mm-bearer-qmi.c

@@ -218,7 +218,7 @@ build_start_network_input (ConnectContext *ctx)
 
     input = qmi_message_wds_start_network_input_new ();
 
-    if (ctx->apn)
+    if (ctx->apn && ctx->apn[0])
         qmi_message_wds_start_network_input_set_apn (input, ctx->apn, NULL);
 
     if (ctx->auth != QMI_WDS_AUTHENTICATION_NONE) {