Some time ago the invariant for CRC cordreps was relaxed to allow for nullptr values on empty cords with an explicit empty CRC value. The CordzInfo analysis never checked for nullptr values causing cord sampling to crash if the sampling happened to include a (very unlikely) empty Cord value.

PiperOrigin-RevId: 558202613
Change-Id: Ib0e1eadd08047167e4df5d3035b36dca2c285a0d