How to check empty value in cmd file?

Use IF DEFINED variable without the percent signs around variable.

Tested in XP (32bit) and Win7 x64:

SET PASSWORD=
IF DEFINED PASSWORD (echo PASSWORD = %PASSWORD%) ELSE (echo PASSWORD is empty or undefined)
IF DEFINED USERNAME (echo USERNAME = %USERNAME%) ELSE (echo USERNAME is empty or undefined)

The following should do it:

if [%PASSWORD%] NEQ [] GOTO MyLabel

For more info, see ss64.com.