Fix Style/DoubleNegation linting error
Fix Style/DoubleNegation linting error

Fixes a Style/DoubleNegation linting error was introduced in the Realex
gateway by changing the statement to `!statement.nil?` from
`!!statement`.

4493 tests, 71886 assertions, 0 failures, 0 errors, 0 pendings,
  0 omissions, 0 notifications
692 files inspected, no offenses detected

pinpayments/active_merchant,Shopify/active_merchant,Simplero/active_merchant,vampirechicken/active_merchant,buttercloud/active_merchant,varyonic/active_merchant,activemerchant/active_merchant,atxwebs/active_merchant,vanboom/active_merchant,curiousepic/active_merchant