fix(filterFilter): fix matching against null/undefined

[gkalpak]

Included fixes:

• Do not convert null/undefined to strings for substring matching in
  non-strict comparison mode. Prevents null/undefined from being
  matched against e.g. 'u'.
• Let null (as a top-level filter expression) match "deeply" (as do
  booleans, numbers and strings).
  E.g. let filterFilter(arr, null) match an item like {someProp: null}.

Closes #11432

_{1st and 3rd commits are refactorings (not important for the fix).
The 2nd commit contains the actual fix(es).
I kept them separate, for easier reviewing or eclectic merging.I can squash them if you think it is better.}

[pkozlowski-opensource]

@gkalpak it looks good to me after taking a quick look, but one of the commits upsets TravisCI (jsce). Could you please have a look?

[gkalpak]

@pkozlowski-opensource: I fixed the error and tests are green now.
Shall I merge then (hopefully doing it the right way this time 😄) ?

[pkozlowski-opensource]

@gkalpak yeh, LGTM

[Narretz]

One question, are these regression fixes or is this something that never worked correctly before?

[gkalpak]

@Narretz, they are regression fixes:

Do not convert null/undefined to strings for substring matching in non-strict comparison mode.
Prevents null/undefined from being matched against e.g. 'u'.

Broken since v1.3.6 / v1.4.0-beta.1.

Let null (as a top-level filter expression) match "deeply" (as do booleans, numbers and strings).
E.g. let filterFilter(arr, null) match an item like {someProp: null}.

Broken since v1.3.15 / v1.4.0-beta.6.

[gkalpak]

@pkozlowski-opensource / @petebacondarwin, can you please take a look to make sure I got this right this time 😄

[pkozlowski-opensource]

@gkalpak commits look fine and TravisCI is green so things look in order :-)