Change error wording for empty \x{}

Change error wording for empty \x{}

An empty \x{} is unfortunately legal (returning a NUL) except in the
scope of "use re 'strict'".  Since this is an experimental feature,
things like wording changes are allowed.  It is unlikely anyone is
relying on the precise wording of this fatal error under 'strict', and
now all the messages for similar errors are similarly worded.