IE allows undefined to be changed, so comparing to undefined is not gauranteed to be correct in IE.
Testing typeof v !== 'undefined' will always give the correct result.

Ext.isDefined(v){...} uses typeof v !== 'undefined'
Ext.isEmpty(v, allowBlank){...} uses v===undefined

Chrome does not allow undefined to be changed so both functions always return the correct result in Chrome; however, IE allows undefined to be changed (IE should not allow this, but it does).

//TEST CODE:
var t0="dishwasher"
if(undefined="dishwasher"){
//an incorrectly writtten IF corrupts undefined
}
var t1="dishwasher";
var t2;
console.log("t1 isDefined:"+ Ext.isDefined(t1));
console.log("t1 isEmpty: "+ Ext.isEmpty(t1));
console.log("t2 isDefined:"+ Ext.isDefined(t2));
console.log("t2 isEmpty: "+ Ext.isEmpty(t2));

IE RESULTS:

t1 isDefined:true <<< this is correct
t1 isEmpty: true <<< this is wrong!!! it is not empty, it contains 'dishwasher'
t2 isDefined:false <<<this is correct
t2 isEmpty: false <<<this is wrong!!! if t2 is undefined then isEmpty() should return true!!!

CHROME RESULTS:
t1 isDefined:true
t1 isEmpty: false
t2 isDefined:false
t2 isEmpty: true

Please consider changing isEmpty() to use isDefined() or to use typeof like isDefined() does and review all EXT code for use of v===undefined which is unsafe test in IE.