Will this be true if the value is null?

Code:
 
$bbexternals = ReadValue ("HKEY_LOCAL_MACHINE\SOFTWARE\Quest Software\BigBrother\bbnt\Externals","")
If $bbexternals


I want to find if there is a string, and if there is append it if the new value doesn't exist. I'm just not sure how to check if a value is null.
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I could have made a neat retort but didn't, for I was flurried and didn't think of it till I was downstairs.
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