Small fixes for faceting and formula_override problems

[LMBradford]

These commits stop the analysis from breaking if

• the formula_override config parameter is null
• facets parameter is null
• the facet value(s) aren't in the contrasts file (ex. facets are "A" and "B"; current code requires that the contrasts be something like:
  A_10 A_0

[LMBradford]

This needs to be merged before changes in the validation branch will work properly.

[mattjmeier]

All good!