Noissue - Remove unnecessary references to undefined

undefined is a global variable of type "undefined". Access to undefined, the variable, is significantly slower than just using null instead. This is why we use the check `typeof foo == "undefined"` rather than `foo == undefined` all over the place.

This patch removes the last remaining references to undefined.

[Manish Jethani, 2018-06-12 04:33:25]

Patch Set 1

https://codereview.adblockplus.org/29804571/diff/29804572/lib/filterStorage.js
File lib/filterStorage.js (left):

https://codereview.adblockplus.org/29804571/diff/29804572/lib/filterStorage.j...
lib/filterStorage.js:769: this.wantObj = undefined;
There's no reason to specifically assign this to undefined, it can assigned to
null instead.

https://codereview.adblockplus.org/29804571/diff/29804572/lib/synchronizer.js
File lib/synchronizer.js (left):

https://codereview.adblockplus.org/29804571/diff/29804572/lib/synchronizer.js...
lib/synchronizer.js:290: return undefined;
Totally redundant, a function returns undefined by default.

[Manish Jethani, 2018-06-12 07:47:51]

Patch Set 2: Use null in sparse array

https://codereview.adblockplus.org/29804571/diff/29805555/lib/content/elemHid...
File lib/content/elemHideEmulation.js (left):

https://codereview.adblockplus.org/29804571/diff/29805555/lib/content/elemHid...
lib/content/elemHideEmulation.js:190: regexpRegexp.exec(text) || [undefined,
textToRegExp(text)];
This also doesn't have to be undefined, it should just be null.

[sergei, 2018-06-12 09:12:31]

LGTM