Material property layout/table

[hchandra]

When none is entered, the cell should be left blank instead of the default 0 (zero). 

For example, the electrical resistivity automatically shows 0 (zero) when it is intentionally left blank/undefined.

This is very confusing since zero resistivity (to me) means a superconducting material. 

FYI: my material is an insulator with unknown electrical resistivity. I am trying to run some Joule heating simulation and do not want current to flow through this material which is in contact with a copper conductor.