Fix inconsistent null handling between array and single object version RAM estimation

[timgrein]

When checking an array of Accountable there's an explicit null check avoiding NPEs, which would happen, when calling accountable.ramBytesUsed().

For the single object version sizeOf(Accountable accountable) this is not the case, therefore aligning the implementations.

[timgrein]

I cannot add people as reviewers unfortunately. @ChrisHegarty can you maybe take a look?

[ChrisHegarty]

Did you encounter this in a real situation, or why the need to allow null? ( I'm just raising the question as I believe this api has been like this for a long time, and want to know if it's a real issue or just for consistency's sake )

[timgrein]

Did you encounter this in a real situation, or why the need to allow null? ( I'm just raising the question as I believe this api has been like this for a long time, and want to know if it's a real issue or just for consistency's sake )

@ChrisHegarty Yes I encountered it while working on a new CircuitBreaker for the inference API in Elasticsearch, where I've used it for RAM estimation. It felt inconvenient that I had to include a null-check, when I'm working with a single Accountable object vs. without a null-check when using an array.