Use explicit empty spans to skip conversion from arrays

[adamreeve]

Small tidy up following on from #600 where I noticed there were a few places where we create an empty array with Array.Empty<TPhysical>() just to pass to a method that expects a ReadOnlySpan. To skip the conversion from an array to span and make it clearer what types are being used I've changed these to create an empty ReadOnlySpan directly. I also changed some places that pass null to use ReadOnlySpan.Empty or Span.Empty too for consistency.