Use built-in methods for nulls_first, nulls_last

[stereobooster]

PR for #1373. PR not ready for merge - let's discuss. I didn't rename options or tests for now

Summary:

• Works for all databases except MySQL in Rails 7
• Works for PostgreSQL in Rails 6

[scarroll32]

Related to #1184

[deivid-rodriguez]

@stereobooster Makes sense to me to use built-in Rails methods, better than the other two options you presented at #1373. Can you do the necessary renamings/test fixes?

[stereobooster]

@deivid-rodriguez yes I can. The problem is that it is kind of braking change. What shall I do about it?

[deivid-rodriguez]

Can you explain the breaking change more? Is it because some supported Rails versions don't yet have these?

[stereobooster]

Option is called postgres_fields_sort_option, but it would work for other databases as well. Shall I leave it as is?

[deivid-rodriguez]

Oh, I didn't notice the option name. We'll be releasing Ransack 3 soon, we I'd rename the option since otherwise it's going to be very confusing.