ARROW-3805: [Gandiva] Handle null validity bit-map in if-else

[pravindra]

• added a bitmap fn that returns true if there is no bitmap
• added more tests

[codecov-io]

Codecov Report

Merging #2980 into master will increase coverage by 1.02%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #2980      +/-   ##
==========================================
+ Coverage   86.64%   87.67%   +1.02%     
==========================================
  Files         491      423      -68     
  Lines       69393    63974    -5419     
==========================================
- Hits        60124    56087    -4037     
+ Misses       9177     7887    -1290     
+ Partials       92        0      -92

Impacted Files	Coverage Δ
cpp/src/gandiva/llvm_generator.h	100% <ø> (ø)	⬆️
cpp/src/gandiva/annotator.cc	97.95% <ø> (ø)	⬆️
cpp/src/gandiva/llvm_generator.cc	76.41% <100%> (+0.16%)	⬆️
cpp/src/plasma/thirdparty/ae/ae.c	72.03% <0%> (-0.95%)	⬇️
rust/src/record_batch.rs
go/arrow/array/table.go
rust/src/array.rs
go/arrow/math/uint64_amd64.go
go/arrow/internal/testing/tools/bool.go
go/arrow/array/bufferbuilder.go
... and 62 more

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[pcmoritz]

LGTM, thanks for fixing this :)

[xhochy]

+1, LGTM