Personally, I do not accept a whitespace ($STR3) as being 'not empty'.
When a variable that only contains whitespaces is passed onto a parameter, it will often error that the parameters value may not be '$null', instead of saying it may not be a whitespace, some remove commands might remove a root folder instead of a subfolder if the subfolder name is a "white space", all the reason not to accept a string containing whitespaces in many cases.
I find this is the best way to accomplish it:
$STR1 = $null
IF ([string]::IsNullOrWhitespace($STR1)){'empty'} else {'not empty'}
Empty
$STR2 = ""
IF ([string]::IsNullOrWhitespace($STR2)){'empty'} else {'not empty'}
Empty
$STR3 = " "
IF ([string]::IsNullOrWhitespace($STR3)){'empty !! :-)'} else {'not Empty :-('}
Empty!! :-)
$STR4 = "Nico"
IF ([string]::IsNullOrWhitespace($STR4)){'empty'} else {'not empty'}
Not empty