fix: browser cache is always considered stale if .modifyVars wasn't set

[balpha]

The problem this fixes is the following: Assume modifyVars is not used, i.e. it's null.

In cache.setCSS, the :vars cache item is not set, because modifyVars is falsy in this if statement.

Therefore, in cache.getCSS, we're retrieving a non-existent item from the cache, and so vars is null.

But modifyVars, which is null at function call time, is them defaulted to {}. In particular, in the check whether the cache is up-to-date, modifyVars is never falsy.

Thus !modifyVars && !vars is never truthy, and the up-to-date decision comes down to
JSON.stringify(modifyVars) === vars, which is "{}" === null and thus false.

So the locally cached version is never used.

[matthew-dean]

@balpha Good catch! Thanks!