Fix multipleOf bugs in Number type

[bertramakers]

Fixes the multipleOf validation crashing when trying to use the modulo operator on a value that is zero, and an InvalidArgumentException when calling multipleOf() with null to reset it.

Edit: To clarify, 0 is considered to be a multiple of any multipleOf value by JSON schema, see for example https://www.jsonschemavalidator.net/s/lPabCUQd. This is also confirmed on https://www.learnjsonschema.com/2020-12/validation/multipleof/ (See examples: "Valid - 0 is a multiple of 4.1")

[bertramakers]

@tobyzerner It would be useful if this could also be merged into the openapi branch once approved, since we're using that for now until the OpenAPI generator is finished.

[bertramakers]

Closing this because the issue I was having was actually that the modulo operator (%) doesn't support floats as a divider, while I was using 0.01 as multipleOf. I tried fixing this by using fmod() instead but because 0.01 is hard to represent accurately as a floating point number it doesn't work as expected. So I'm closing this since it doesn't solve the original issue and I will have to use a workaround that is not generic enough to implement in this class since it will be specific to some of our types where the multipleOf is 0.01 specifically. Nevertheless it's an interesting edge case that could benefit from some more investigation down the line I think.

Example: https://onlinephp.io/?s=s7EvyCjg5eLlKkssik8pzS3QSMvNT9EwNDDQUTDQM9TUtAYA&v=8.0.28