allow has_equal_ast to work, even if no solution_ast

[machow]

Currently, when there's an AST misparse, then ast based SCTs are quietly skipped. This was done since we can't guarantee that for every parser will handle all student submissions handed to it. However, has_equal_ast can be adapted to work without a solution ast.

(In general, it is not clear if checking the submission and solution in an SCT is beneficial--if people will be using the exercise validator)