[lending] Can't withdraw small amount to close position

[tomjeatt]

See here:
https://discord.com/channels/745259537707040778/1152617257982242866

Reported by Philip. When trying to withdraw a tiny amount to close a position (i.e. reduce token balance to 0) the UI validation appears to be treating the small token balance as zero, and so blocks the transaction.

We need to adjust this to allow the smallest possible amount to be withdrawn.