Parser followup (#2377)

* set pipe to empty string on empty closure

* fix missing parser flags and scope

* add closure serializing

* add enum provider function to slash command arguments

* add enum providers for /bg, /ask, and /go

* fix index out of bounds returning undefined

* keep whitespace as is in mixed unnamed args (string+closure)

* add _hasUnnamedArgument to named arguments dictionary

* allow /var key=x retrieval

* add enum provider to /tag-add

* fix typo (case)

* add option to make enum matching optional

* add executor to enum provider

* change /tag-add enum provider to only show tags not already assigned

* add enum provider to /tag-remove

* fix name enum provider excluding groups

* remove void from slash command callback return types

* Lint undefined and null pipes

* enable pointer events in chat autocomplete

* fix type hint

---------

Co-authored-by: LenAnderson <Anderson.Len@outlook.com>
Co-authored-by: Cohee <18619528+Cohee1207@users.noreply.github.com>

public/scripts/slash-commands/SlashCommandScope.js

@@ -92,7 +92,7 @@ export class SlashCommandScope {
                     v = v[numIndex];
                 }
                 if (typeof v == 'object') return JSON.stringify(v);
-                return v;
+                return v ?? '';
             } else {
                 const value = this.variables[key];
                 return (value === '' || isNaN(Number(value))) ? (value || '') : Number(value);