Change subkey references to return null if not found rather than runtime error

[toggledbits]

There are times when data could be missing from an API response, so to give expressions a clean way to handle that, subkey references should be changed so that they return null rather than throw a runtime error. This null result could then be detected, such as by the if() function, to provide a default expression result in the absence of the data sought.

[toggledbits]

Implemented in develop branch--updated version of luaxp in place (the actual changes required to support this were in luaxp, not SiteSensor itself).

[toggledbits]

Released in v1.8