Add isValid to SysReadHandle

Merged Tadej Novak requested to merge tadej/athena:syshandle-valid into 23.0

Add isValid to SysReadHandle similar as it's already available in the standard ReadHandle. Not sure how much it makes sense to add it to the SysWriteHandle.

Keeping draft for now as it needs a bit of testing (it's actually relatively hard to break CP algorithms to have invalid inputs, but some special analysis workflows may require it).

Opening in 23.0 as it's kind of core infrastructure that it would be good to be in sync.

/cc @krumnack @omajersk

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