TT#71952 Handle undefined X_f.alias keys in get_cdr_export_fields

The alias key is defined as optional in the config.yml schema. This was
producing perl warnings in the templates files:

  ,---
  Argument "" isn't numeric in numeric gt (>) at /usr/lib/ngcp-ngcpcfg/get_cdr_export_fields line 45.
  Argument "" isn't numeric in numeric gt (>) at /usr/lib/ngcp-ngcpcfg/get_cdr_export_fields line 45.
  Argument "" isn't numeric in numeric gt (>) at /usr/lib/ngcp-ngcpcfg/get_cdr_export_fields line 45.
  Argument "" isn't numeric in numeric gt (>) at /usr/lib/ngcp-ngcpcfg/get_cdr_export_fields line 45.
  2020-01-14 15:02:30 sp1: Generating /etc/ngcp-cdr-exporter/event-exporter.conf: OK
  `---

Change-Id: I46084fbd25e9c88c8cdddb7511c378ac06135632

lib/get_cdr_export_fields

@@ -202,7 +202,7 @@ END;
 
 out.joins = [];
 FOREACH X_f IN argv.export_joins;
-  IF X_f.alias.length > 0;
+  IF X_f.alias && X_f.alias.length > 0;
     X_table_spec = X_f.table _ ' ' _ X_f.alias;
   ELSE;
     X_table_spec = X_f.table;
@@ -226,4 +226,4 @@ FOREACH X_f IN argv.export_conditions;
   out.conditions.push("{ '" _ X_condition_column _ "' => { '" _ X_f.operand _ "' => '" _ X_f.compare_value _ "' } }");
 END;
 
--%]
+-%]