feat(eslint-plugin): [strict-boolean-expressions] Add allowNullable option

[phaux]

Add allowNullable option to strict-boolean-expressions rule. It should work the same as allow-undefined-union + allow-null-union from tslint.

Fixes #698

[typescript-eslint]

Thanks for the PR, @phaux!

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[codecov]

Codecov Report

Merging #794 into master will decrease coverage by <.01%.
The diff coverage is 93.75%.

@@            Coverage Diff             @@
##           master     #794      +/-   ##
==========================================
- Coverage   94.06%   94.06%   -0.01%     
==========================================
  Files         113      113              
  Lines        4937     4952      +15     
  Branches     1378     1384       +6     
==========================================
+ Hits         4644     4658      +14     
  Misses        166      166              
- Partials      127      128       +1

Impacted Files	Coverage Δ
...int-plugin/src/rules/strict-boolean-expressions.ts	97.14% <93.75%> (-2.86%)	⬇️

[Retsam]

I looked into adding a similar option to this rule awhile back, but ended up writing a new rule instead, which is still in PR (#699). If your intent is similar to mine (allow conditions that aren't strictly booleans, but avoid pointless conditions like true &&), you might check out that rule in PR; might be more straightforward than adding options to this rule.

[phaux]

@Retsam My use case is that I want to enforce being explicit to make my code more clear in situations like these:

• arr.length → arr.length > 0
• obj.prop → obj.prop != null or "prop" in obj
• str → str.length > 0 or str != ""

I needed this option so I don't have to write condition == true when condition is already a boolean but is also optional.

I don't care about unnecessary conditions at all. I sometimes do "unnecessary" checks on purpose:

function example(opts: Opts) {
  // its required but I also want to check at runtime anyways
  if (opts.someRequiredOpt == null) throw Error("...")
}

[bradzacher]

LGTM.

I think it's okay to add some more flexibility to this rule.
Thanks for doing this.

[JamesHenry]

Thanks for contributing, @phaux!