How do I restrict a user from entering "0" Hours on the Time Card

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The client does not want employees to enter 0 hours on the Time Card. I tried writing a FF for it.

But even when the user does not enter any value the FF is throwing the message that 0 hour is entered. On checking the log, I noticed that for days where nothing is entered 0 is getting passed to the Fast Formula.

Any ideas on how to write this rule

 

Thanks

SG