Guard on null ExpressionListPtr in alias manager

Summary: Noticed this while working on D1523396.
Calling an alias method like is_array() with no args
results in a segfault in repo mode when ep->getCount()
tries to dereference a nullptr.

Guard against it and let the normal course of the runtime
raise the appropriate error.

Reviewed By: @markw65

Differential Revision: D1524843
is_<type>() functions should be ParamCoerceModeFalse

Summary: But weirdly is_numeric()/is_scalar()/is_callable() return NULL.

Closes #3546

Reviewed By: @JoelMarcey

Differential Revision: D1523396