analyzer: fix some svalue::dump_to_pp implementations

gcc/analyzer/ChangeLog:
	* svalue.cc (region_svalue::dump_to_pp): Support NULL type.
	(constant_svalue::dump_to_pp): Likewise.
	(initial_svalue::dump_to_pp): Likewise.
	(conjured_svalue::dump_to_pp): Likewise.  Fix missing print of the
	type.

Signed-off-by: David Malcolm <dmalcolm@redhat.com>