Preserve State in State Refinements without Postcondition

[rcosta358]

Closes #51.

Fixes a bug where @StateRefinement annotations with a from condition but no to condition were incorrectly treated as state transitions. This caused LiquidJava to weaken object state after calls that should only check a precondition, losing precise information needed by later calls.

Root cause

When a state refinement had from != null and to == null, the verifier created a post-state by copying from into to, which turned a pure state check into an actual transition. Fixed this by removing this part of the logic.

This means that, for example, a method that should only require x(this) > 0 could overwrite a more precise state such as x(this) == 2 with the weaker predicate x(this) > 0.