Add isNotEmpty extension

[PaulTaykalo]

From one point of view, this is a simple sugar-like extension.

Pros:

• Some expressions that are using isEmpty calls will look like a natural language
• No going back and forth when parsing some complex conditional expressions

Cons:

• Some linting rules for the framework itself can break
• There's no lining rule that will force this usage later.

[SwiftLintBot]

	2 Warnings
⚠️	Please include a CHANGELOG entry to credit yourself! You can find it at CHANGELOG.md.
⚠️	This PR may need tests.

	12 Messages
📖	Linting Aerial with this PR took 1.94s vs 1.98s on master (2% faster)
📖	Linting Alamofire with this PR took 2.75s vs 2.76s on master (0% faster)
📖	Linting Firefox with this PR took 9.36s vs 9.38s on master (0% faster)
📖	Linting Kickstarter with this PR took 15.52s vs 15.58s on master (0% faster)
📖	Linting Moya with this PR took 1.33s vs 1.36s on master (2% faster)
📖	Linting Nimble with this PR took 1.37s vs 1.36s on master (0% slower)
📖	Linting Quick with this PR took 0.61s vs 0.68s on master (10% faster)
📖	Linting Realm with this PR took 4.52s vs 4.5s on master (0% slower)
📖	Linting SourceKitten with this PR took 1.02s vs 1.07s on master (4% faster)
📖	Linting Sourcery with this PR took 7.3s vs 7.32s on master (0% faster)
📖	Linting Swift with this PR took 11.01s vs 11.06s on master (0% faster)
📖	Linting WordPress with this PR took 17.63s vs 17.65s on master (0% faster)

Here's an example of your CHANGELOG entry:

* Add `isNotEmpty` extension.  
  [PaulTaykalo](https://github.com/PaulTaykalo)
  [#issue_number](https://github.com/realm/SwiftLint/issues/issue_number)

note: There are two invisible spaces after the entry's text.

Generated by 🚫 Danger

[codecov-io]

Codecov Report

Merging #3387 into master will decrease coverage by 0.00%.
The diff coverage is 95.32%.

@@            Coverage Diff             @@
##           master    #3387      +/-   ##
==========================================
- Coverage   90.50%   90.49%   -0.01%     
==========================================
  Files         420      420              
  Lines       20552    20555       +3     
==========================================
+ Hits        18601    18602       +1     
- Misses       1951     1953       +2     

Impacted Files	Coverage Δ
...intFramework/Documentation/RuleDocumentation.swift	0.00% <0.00%> (ø)
Source/SwiftLintFramework/Models/LinterCache.swift	63.29% <0.00%> (ø)
...onfigurations/UnusedDeclarationConfiguration.swift	13.63% <0.00%> (ø)
...wiftLintFramework/Extensions/Array+SwiftLint.swift	100.00% <100.00%> (ø)
...ntFramework/Extensions/Configuration+Parsing.swift	93.75% <100.00%> (ø)
...LintFramework/Extensions/SwiftLintFile+Regex.swift	96.74% <100.00%> (-0.47%)	⬇️
Source/SwiftLintFramework/Models/Command.swift	97.43% <100.00%> (ø)
...urce/SwiftLintFramework/Models/Configuration.swift	83.53% <100.00%> (ø)
Source/SwiftLintFramework/Models/Linter.swift	84.93% <100.00%> (ø)
Source/SwiftLintFramework/Protocols/Rule.swift	64.83% <100.00%> (ø)
... and 59 more

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[jpsim]

Thanks!