Including imported module first as cli arg with generic interface leads to bad error message

[expipiplus1]

Following on from this: #5660

If we make I a generic interface then we get a very bad error message

nullType.slang:

import nullTypeModule;

struct S { }

extension S : I<float> { }

nullTypeModule.slang:

interface I<float> { } 

slangc -stage compute tests/bugs/nullTypeModule.slang tests/bugs/nullType.slang

(0): error 30060: expected a type, got a '<null>'