[CS2] Fix #4629: interpolations (whether in strings or CSX tags) with only comments

[GeoffreyBooth]

Fixes #4629.

An optimization in interpolated strings (which CSX tags essentially are) ignored empty interpolations: "a#{}b" compiled to just `ab`, not `a${}b`. This meant that an interpolation that had nothing but comments, like "a#{### comment ###}b", was also getting output as just `ab`, with the comment lost.

This PR preserves both the optimization and the comments. If an “empty” interpolation has no comments, the current behavior remains. However if it has comments but no other tokens, a zero-length JS token is created in the interpolation to hold the comments; and the optimization isn’t performed.

[lydell]

This looks good for the JSX case! 👍

Unfortunately, "a#{###test###}b" becomes
`a${/*test*/}b`, which is invalid JS.

[GeoffreyBooth]

Huh. I had no idea `a${/*test*/}b` is invalid JS. How weird! I wonder why?

So what should we do for strings? Compile to `a${/*test*/''}b`?

[lydell]

Empty interpolations are invalid in JS. I don't know why but that's how it is. I think your solution with the empty string looks good.

[GeoffreyBooth]

Done!

[connec]

We should probably make "a#{###test###}b" invalid CS.

[lydell]

@connec You mean empty interpolations in general?

[connec]

Yeh, I don't think it's useful to allow empty interpolations.

That said, I think I misunderstood the outcome of the PR - treating empty interpolations as '' seems reasonable too.

[GeoffreyBooth]

It would be a breaking change to no longer allow "a#{###test###}b", not that that would be so terrible. But I think since the solution is so simple, we might as well improve on template literals a little 😄