media: fix return value for mediaTypeToString

The function mediaTypeToString never checked for nullptr
return values, which could potentially lead to crashes.
This patch ensures it always returns a valid string for
all defined MediaType values.

Change-Id: I81ef8876e1f57265584fae02cdfaff2894ff9f8c

src/media/media_attribute.cpp

@@ -136,7 +136,9 @@ MediaAttribute::mediaTypeToString(MediaType type)
         return libjami::Media::MediaAttributeValue::AUDIO;
     if (type == MediaType::MEDIA_VIDEO)
         return libjami::Media::MediaAttributeValue::VIDEO;
-    return nullptr;
+    if (type == MediaType::MEDIA_NONE)
+        return libjami::Media::MediaAttributeValue::SRC_TYPE_NONE;
+    return "UNKNOWN";
 }
 
 bool