Allow optional block for #find_or_create_by

[fugakkbn]

ActiveRecord::Relation#find_or_create_by(!) type definition requires a block.
If we try to run without blocks, we will get the following error:

[error] The method cannot be called without a block
│ Diagnostic ID: Ruby::RequiredBlockMissing
│
└     users.find_or_create_by!(nickname: fugakkbn)

However, since there are cases where it is called without a block, this PR makes the change to make the block optional.

[pocke]

Thanks!