Fix float argument check in minetest.set_timeofday()

[Zughy]

• Goal of the PR
  Avoid an engine assumption crash
• How does the PR work?
  I've changed the check, now it doesn't make the whole client crash anymore (only the game)
• Does it resolve any reported issue?
  closes Many Lua C functions are missing type checks #1794 (I've tried all the listed functions, they crash as intended except set_timeofday)

I guess there are other functions that might not work as intended with weird values, but testing them all is a suicide. I suggest to solve them on the go, avoiding keeping an issue open - especially because the listed ones are all solved

To do

This PR is Ready for Review.

How to test

Make it crash with

minetest.register_on_joinplayer(function(player, last_login)
    minetest.set_timeofday(-1)
end)

[HybridDog]

I've tested it with the /time nan chatcommand; a hard crash no longer happens.

[paramat]

I did not test but code looks fine 👍