Both osl_add/removeSignalHandler apparently handle !handler gracefully (the
former even including explicit code for that case, which doesn't fit with having
an assert), and before bca760c2 "tdf#43157 -
osl: convert OSL_ASSERTs to assert in signalshared.cxx" those asserts were
merely OSL_ASSERTs.  So keep this published API accepting !handler (even if the
documentation is silent on this).

Change-Id: I63512916a66a2a58b932174ec5d92ce612c236a2