Fluent Logging: Allow nullable values

[Zack-Freedman-Thoughtworks]

References: #400 (comment)

Example:

Example:

logger.atError {
    this.message = "Failed to make a good example: ${throwable.message}."
    this.cause = throwable
    this.payload = mapOf(
        "itemOne" to headers.getNullableThing(),
    )
}

Allows the value in the payload to be nullable. It seems that the code referencing the payloads are are written in Java so it is harder to know if this will lead to a null pointer exception or other troubles with possible loggers but the tests pass.

[oshai]

From code checks I did I think the Java frameworks allow values to be null.

[oshai]

Thanks!