Fix an error where 'false' string is displayed when we add a new parameter in the Parameters tab,

also clear the old value when the user changes the parameter name.

Corrected the implementation of BooleanCellFormatter.

Fixes #4753

web/pgadmin/browser/server_groups/servers/static/js/variable.js

@@ -87,7 +87,8 @@ function(gettext, _, $, Backbone, Backform, Backgrid, Alertify, pgAdmin, pgNode)
         setTimeout(function() {
           self.columns.each(function(col) {
             if (col.get('name') == 'value') {
-
+              // Reset old value
+              self.model.set({'value': undefined}, {silent:true});
               var idx = self.columns.indexOf(col),
                 cf = col.get('cellFunction'),
                 cell = new (cf.apply(col, [self.model]))({

web/pgadmin/static/js/backgrid.pgadmin.js

@@ -1890,7 +1890,8 @@ define([
     },
   });
 
-  Backgrid.BooleanCellFormatter = _.extend(Backgrid.CellFormatter.prototype, {
+  var BooleanCellFormatter = Backgrid.BooleanCellFormatter = function() {};
+  _.extend(BooleanCellFormatter.prototype, {
     fromRaw: function (rawValue) {
       if (_.isUndefined(rawValue) || _.isNull(rawValue)) {
         return false;