[V3/Bank] Change withdraw to accept 0.

[Redjumpman]

Type

• Bugfix
• Enhancement
• New feature

Description of the changes

This change makes it so withdraw_credits will accept 0 for the amount. It currently raises a ValueError. When a cog requires credits to be used for some functionality an additional check has to be added for 0 (Essentially when someone wants to make it free). For example:

if amount == 0:
    return True

 try:
    await bank.withdraw_credits(member, amount)
except ValueError:
    return False
else:
    return True

This change will eliminate that leading if statement. This won't adversely effect someone's balance, because under the operation is only subtracting 0. Zero is the identity element in subtraction.