Give a empty non-null empty sourcemap warnings

[hyf0]

What problem does this feature solve?

.

What does the proposed API look like?

See https://github.com/rollup/rollup/blob/28546b5821efcb72c2eb05f422d986524647a0e3/src/utils/transform.ts#L73-L74 for details.

[shulaoda]

Try it. 🙌

[underfin]

Here the null will not push into sourcemap chain, but undefined will be. The remapping result is same if has a empty sourcmap exit. Different is the undefined will be give a warning, see here.

It meaning it only need to do if here return non-null empty sourcemap. It should be added into transfrom and renderChunks hook.