Cop idea: detect use of #find in control flow

[Drenmi]

Is your feature request related to a problem? Please describe.

Quite often I see people write code with the assumption that #find will return a falsey result when failing to find a record.

Describe the solution you'd like

Perhaps we can have a cop that detects at least the most basic cases of use of #find in conditionals, e.g.:

if foo = Foo.find(params[:id])

[koic]

This cop should handle if foo = Foo.find_by!(conditions) as well, right?

[koic]

I will close this issue because it can be considered together with rubocop/rails-style-guide#333.