Blank Password is accepted in the create command

[nus-se-bot]

When using the create command to create an account, the command
create u/abcde pw/ will run, which is undocumented behaviour (accepting empty string as password). If passwords should be optional, maybe it would be better to indicate usage as create u/USERNAME [pw/PASSWORD], since the password field is actually optional, and blank passwords is not in the spirit of having a password field.

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Reported by: @zioul123
Severity: Low

_{[original: nus-cs2103-AY1819S1/pe-dry-run#142]}