grgsm_livemon: Change the type of several parameters to string

It seems gnuradio 3.8 does not like parameters with type None and does
not put them as command line arguments

Change-Id: Id67e37e826aa1cc14640d57207a92def682f11e3
diff --git a/apps/grgsm_livemon.grc b/apps/grgsm_livemon.grc
index 2f6d29c..4e07e88 100644
--- a/apps/grgsm_livemon.grc
+++ b/apps/grgsm_livemon.grc
@@ -105,7 +105,7 @@
     hide: none
     label: Device Arguments
     short_id: ''
-    type: ''
+    type: str
     value: '""'
   states:
     bus_sink: false
@@ -178,7 +178,7 @@
     hide: none
     label: IP or DNS name of collector point
     short_id: ''
-    type: ''
+    type: str
     value: 'localhost'
   states:
     bus_sink: false
@@ -195,7 +195,7 @@
     hide: none
     label: UDP port number of collector
     short_id: ''
-    type: ''
+    type: str
     value: '4729'
   states:
     bus_sink: false
@@ -912,7 +912,7 @@
     hide: none
     label: UDP server listening port
     short_id: ''
-    type: ''
+    type: str
     value: '4729'
   states:
     bus_sink: false