[if exists]{PARAM} – else clause fails unless param sent as blank string

[codemonkeynorth]

[if exists]{PARAM}
[show]The first parameter was passed
[else]The first parameter wasn't passed
[/if]

the else only works correctly if i send as [myshortcode param=""]

if i just use [myshortcode] then [if exists] returns true.

see https://wordpress.org/support/topic/if-existsparam-else-clause-fails-unless-param-sent-as-blank-string/

thanks J