Fix LauFormulaPar to follow change in behaviour of TFormula

Summary:
Dimension returned by variable-less TFormula is now (sensibly) 0 rather than 1.
The dummy variable array passed to EvalPar is also no longer required.

Test Plan:
Have created a LauFormulaPar and verified it gives the correct results as the
values of the underlying parameters change.

Reviewers: johndan, mwhitehe

Reviewed By: johndan, mwhitehe

Tags: Laura

Maniphest Tasks: T129

Differential Revision: https://phab.hepforge.org/D42