Make CronExpression equality operators accept null without triggering nullable warnings

[Nechepso]

Since commit 666db5c shipped in version 0.11.0, a CronExpression can no longer be compared with null using the overloaded equality operators without a CS8625 nullable warning being produced. The following code shows the issue:

CronExpression? a = /* A CronExpression that may or may not be null. */;
if (a != null) { // <-- This line triggers CS8625: Cannot convert null literal to non-nullable reference type
    // [...]
}

This PR modifies the overloaded equality operators' arguments nullability to accept null values in order to make the previous example compile without warnings.

@odinserj: I also took the liberty to enable NRT annotation/checking in the Directory.Build.props of the tests folder (and treat all nullability-related warnings as errors) to align it with the src folder's one. I tested it and as expected, without the other changes introduced by this PR, the unit tests would not compile anymore. Let me know whether you want to keep it this way or not.

[odinserj]

Awesome, @Nechepso, this is surely expected! Merging and releasing

[odinserj]

Done – https://github.com/HangfireIO/Cronos/releases/tag/v0.11.1

[Nechepso]

Thanks a lot! That was fast. 😄

[odinserj]

Welcome, 28 minutes, thanks to tests and automated pipeline 😅