Do not set nil value to java.util.Properties

[jlahtinen]

This returns easier to understand error message

Before this error in default full rails engine

Failure/Error: ActiveRecord::Migration.maintain_test_schema!

LoadError:
  load error: rails_helper -- java.lang.NullPointerException: null
# .rvm/gems/jruby-9.1.13.0/gems/activesupport-5.2.1/lib/active_support/dependencies.rb:287:in `block in require'

After this

Failure/Error: ActiveRecord::Migration.maintain_test_schema!

RuntimeError:
  password not set
# gems/oracle-enhanced/lib/active_record/connection_adapters/oracle_enhanced/jdbc_connection.rb:129:in `new_connection'```