Export job fails when deselecting all the columns.

Select2 parameter 'first_empty' should be set to false for the controls of type array.
If it is true then on deselecting/clearing all the values, an empty entry is still there. 

Fixes #4751

web/pgadmin/browser/server_groups/servers/databases/schemas/tables/static/js/table.js

@@ -400,8 +400,8 @@ define('pgadmin.node.table', [
           id: 'coll_inherits', label: gettext('Inherited from table(s)'),
           url: 'get_inherits', type: 'array', group: gettext('Columns'),
           disabled: 'checkInheritance', deps: ['typname', 'is_partitioned'],
-          mode: ['create', 'edit'],
-          select2: { multiple: true, allowClear: true,
+          mode: ['create', 'edit'], first_empty: false,
+          select2: { multiple: true, allowClear: true, first_empty: false,
             placeholder: gettext('Select to inherit from...')},
           transform: function(data, cell) {
             var control = cell || this,

web/pgadmin/tools/import_export/static/js/import_export.js

@@ -200,6 +200,7 @@ define([
       select2: {
         multiple: true,
         allowClear: true,
+        first_empty: false,
         placeholder: gettext('Colums for exporting...'),
         preserveSelectionOrder: true,
       },
@@ -251,7 +252,7 @@ define([
       select2: {
         multiple: true,
         allowClear: true,
-        first_empty: true,
+        first_empty: false,
         placeholder: gettext('Not null columns...'),
       },
       helpMessage: gettext('Do not match the specified column values against the null string. In the default case where the null string is empty, this means that empty values will be read as zero-length strings rather than nulls, even when they are not quoted. This option is allowed only in import, and only when using CSV format.'),