Allow NULL for update directives in OpenACC 2.6

Allow NULL for update directives in OpenACC 2.6

A non-present passed-by-reference Fortran optional argument is represented
by a null pointer.  When passed to an update directive, it should be ignored
as variable mappings are not created for null pointers.  This should be
safe as it is not possible to change a non-present argument into a present
one (or vice-versa) in Fortran.

	libgomp/
	* oacc-mem.c (update_dev_host): Return early if the host address
	is NULL.
        * testsuite/libgomp.oacc-c-c++-common/lib-43.c: Remove.
        * testsuite/libgomp.oacc-c-c++-common/lib-47.c: Likewise.

Reviewed-by: Julian Brown <julian@codesourcery.com>
Reviewed-by: Thomas Schwinge <thomas@codesourcery.com>
(cherry picked from openacc-gcc-9-branch commit
b930a8a1ee826ca52081c7c0e88554b3091afb62)