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_headless.grc b/apps/grgsm_livemon_headless.grc
index 4d0c83e..1bea77c 100644
--- a/apps/grgsm_livemon_headless.grc
+++ b/apps/grgsm_livemon_headless.grc
@@ -42,7 +42,7 @@
     hide: none
     label: Device Arguments
     short_id: ''
-    type: ''
+    type: str
     value: '""'
   states:
     bus_sink: false
@@ -133,7 +133,7 @@
     hide: none
     label: IP or DNS name of collector point
     short_id: ''
-    type: ''
+    type: str
     value: 'localhost'
   states:
     bus_sink: false
@@ -150,7 +150,7 @@
     hide: none
     label: UDP port number of collector
     short_id: ''
-    type: ''
+    type: str
     value: '4729'
   states:
     bus_sink: false
@@ -804,7 +804,7 @@
     hide: none
     label: UDP server listening port
     short_id: ''
-    type: ''
+    type: str
     value: '4729'
   states:
     bus_sink: false