run_validators memory optmization

[ericproulx]

This PR change the way validator are created before calling validate. run_validators has always created all the validators before calling validate. In a case where an exception is raised and the validator fail_fast? returns true, the remaining validators won't be validate. Thus, we can optimize the validation process by yielding validator instead of creating all of them at first. It's a small memory optimization and maybe it will speed up a little bit any endpoint that has a validator with fail_fast

In the same file, I also replaced some || [] by &.

[grape-bot]

	1 Warning
⚠️	There're library changes, but not tests. That's OK as long as you're refactoring existing code.

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[dblock]

Nice