Add nilslices analyzer

[k1LoW]

ref: https://google.github.io/styleguide/go/decisions#nil-slices

[github-actions]

Code Metrics Report

	main (a258616)	#38 (cab207a)	+/-
Coverage	63.8%	64.8%	+1.0%
Code to Test Ratio	1:0.1	1:0.1	+0.0
Test Execution Time	6s	7s	+1s

Details

  |                     | main (a258616) | #38 (cab207a) |  +/-  |
  |---------------------|----------------|---------------|-------|
+ | Coverage            |          63.8% |         64.8% | +1.0% |
  |   Files             |             10 |            11 |    +1 |
  |   Lines             |            556 |           603 |   +47 |
+ |   Covered           |            355 |           391 |   +36 |
+ | Code to Test Ratio  |          1:0.1 |         1:0.1 |  +0.0 |
  |   Code              |           1645 |          1769 |  +124 |
+ |   Test              |            128 |           146 |   +18 |
- | Test Execution Time |             6s |            7s |   +1s |

Code coverage of files in pull request scope (67.0% → 67.8%)

Files	Coverage	+/-
analyzer/decisions/nilslices/nilslices.go	76.6%	+76.6%
analyzer/decisions/pkgnames/pkgnames.go	77.8%	0.0%
analyzer/decisions/recvnames/recvnames.go	82.1%	0.0%
analyzer/decisions/repetition/repetition.go	53.8%	0.0%
analyzer/decisions/underscores/underscores.go	78.8%	0.0%
analyzer/decisions/varnames/varnames.go	71.1%	0.0%
analyzer/effective/ifacenames/ifacenames.go	77.8%	0.0%
analyzer/guide/mixedcaps/mixedcaps.go	76.8%	0.0%
config/config.go	0.0%	0.0%

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Reported by octocov