#if( !$myPmtUrl || $myPmtUrl.isEmpty() )
and again you don't need to silence the output in the initial #set, and you don't need the curly braces.
#set( $myPmtUrl = $TriggerObject.paymentConfirmUrl )
#if ( !$myPmtUrl || $myPmtUrl.isEmpty() )
Payment URL is empty
#else
Payment URL is ${$myPmtUrl}
#end