[MdTable] Bug: Cannot sort column if any value is null

[FossPrime]

Steps to reproduce

1. go here https://codesandbox.io/s/5wx36noz4n
2. try to sort by Date of Birth

What is actually happening?

TypeError Cannot read property 'localeCompare' of null

Additional Notes

Seems like an easy fix

[babafeng]

Get the bug too

[cargilltay]

Any update to this issue?