Allow TransactionBuilder to accept `null` for `fee`

## Problem

I'd like to rely on suggested params to populate most of the fields but when overriding the `fee` on the transaction with a `flatFee` the builder errors since both `fee` and `flatFee` are set.

If I try to pass `null` to the `fee` method, it only works in the case of `BigInteger` which may be changed at some point.

## Solution

Allow `fee` method in builder to accept `null` for Integer or Long so we can properly unset the fee parameter while still allowing `suggestedParameters` to be passed instead of individual arguments.