fix (validation): fix issue with validation raising exception if nil

[bradrees]

When trying to validate a BSB, if the value is nil it raises an exception:

undefined method `gsub' for nil

from:

def normalize(str)
  str.gsub(/[^\d]/, '')
end

in bsb.rb

I picked this up in my tests:

expect(account).to validate_presence_of(:bsb)

[davelooi]

Hey @bradrees , excellent find. Thank you for your contribution.