Stringify: allow empty string for indent option

[fritz-c]

In the existing implementation, empty strings used in the indent option are treated as falsy and therefore ignored in the following fallback:

css/lib/stringify/identity.js

Line 253 in b7f3cb6

return Array(this.level).join(this.indentation || ' ');

This PR resolves that issue, and adds tests for behavior surrounding the indent option.

[fritz-c]

@SlexAxton Apologies for the ping, but I think you will find this bugfix PR limited in scope, with non-breaking changes and newly added tests to confirm the proper behavior. I would appreciate it if you could review it.

[fritz-c]

@tj Apologies for the ping, but I think you will find this bugfix PR limited in scope, with non-breaking changes and newly added tests to confirm the proper behavior. I would appreciate it if you could review it.

[solugebefola]

Has anyone taken a look at this PR? I got here from the issue in jest-styled-components @dineshks1 ?

[fritz-c]

@SlexAxton Thank you for merging the PR. Could you make an npm release including the fix so I can complete the PR that depends on this in jest-styled-components?

[cincodenada]

Ha, well well...I came here after taking the same journey as @fritz-c tracking down why my diffs in jest-styled-components were broken, and got as far as re-finding the bug, mostly fixing it, running into this same issue, guessing it was an empty string bug, and coming here to confirm.

Which is to say: seconded, is it possible to get a 3.0.1 release with just this change? jest-styled-components is a widely-used library that is much noisier than necessary due to this bug.