Bug: Allow empty string for New-PodeWebHidden -Value parameter

[thekamilpro]

Currently Value parameter is mandatory and either:

• Fails to start web app if empty string is hardcoded
• Crashes the web app if the value is a variable, which results in being an empty string.

I've had the problem with the latter, where certain information has been passed to the form, with some information being assigned to hidden value - when such information happens to be an empty string, the web app crashes.