mgcp: Allow to omit sending the audio name at all

Equipment like AudioCode appears to get upset when we use a
builtin type and then assign a name to it. Allow to completely
omit the name.
diff --git a/openbsc/tests/mgcp/mgcp_test.ok b/openbsc/tests/mgcp/mgcp_test.ok
index bb803d4..4e27282 100644
--- a/openbsc/tests/mgcp/mgcp_test.ok
+++ b/openbsc/tests/mgcp/mgcp_test.ok
@@ -476,4 +476,5 @@
 Stats: Jitter = 0, Transit = -144000
 Testing multiple payload types
 Testing no sequence flow on initial packet
+Testing no rtpmap name
 Done