This has been brought up in the forum before.
Create symbology using two (or more) attribute table fields, where (current and future) values are unknown, but have a default value set to NULL.
One example could be symbolizing the data this way:
"Null, Null" gets one symbol
"Null, Not Null" gets one symbol
"Not Null, Null" gets one symbol
"Not Null, Not Null" gets one symbol (this could simply be "all other values")
In addition to "not NULL", the general use of logical expressions could be handy.
