Fix GroupTable.memberLabel() to return null for blank labels.

app/src/main/java/org/thoughtcrime/securesms/database/GroupTable.kt

@@ -1278,11 +1278,16 @@ class GroupTable(context: Context?, databaseHelper: SignalDatabase?) :
      * Gets the member label for a specific member in the group, or null if the member is not found.
      */
     fun memberLabel(aci: ACI): MemberLabel? {
-      return decryptedGroup
+      val member = decryptedGroup
         .members
         .findMemberByAci(aci)
         .orNull()
-        ?.let { member -> MemberLabel(member.labelEmoji, member.labelString) }
+
+      return if (member != null && member.labelString.isNotBlank()) {
+        MemberLabel(member.labelEmoji, member.labelString)
+      } else {
+        null
+      }
     }
   }