Replace Nullable with CheckForNull

[uhafner]

CheckForNull is now supported by NullAway as well: see uber/NullAway#397

[codecov-commenter]

Codecov Report

Merging #202 into master will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##             master     #202   +/-   ##
=========================================
  Coverage     88.08%   88.08%           
  Complexity       78       78           
=========================================
  Files             8        8           
  Lines           319      319           
  Branches         47       47           
=========================================
  Hits            281      281           
  Misses           30       30           
  Partials          8        8           

Impacted Files	Coverage Δ	Complexity Δ
src/main/java/edu/hm/hafner/util/PathUtil.java	100.00% <ø> (ø)	21.00 <0.00> (ø)
...n/java/edu/hm/hafner/util/StringContainsUtils.java	100.00% <ø> (ø)	6.00 <0.00> (ø)
src/main/java/edu/hm/hafner/util/Ensure.java	76.61% <100.00%> (ø)	8.00 <0.00> (ø)
src/main/java/edu/hm/hafner/util/TreeString.java	91.30% <100.00%> (ø)	18.00 <3.00> (ø)

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