[5.4] Support amount 0 in Arr::random()

[vlakoff]

Follow-up to #20396 and #20402.

To summarize:

• Supports amount 0 in Arr::random() for consistency with Collection::random()
• Supports "0" as a string, for consistency as the other numbers are accepted as strings.
• A handful of additions to the tests, better this way to prevent any risk of regressions.

ping @browner12

[vlakoff]

Added a commit to remove the redundant "enough items available" validation code.

I was tired of seeing this duplication only because the exception messages were slightly different :)