make FFI_ArrowArray::empty() public

[wangfenjin]

Same as FFI_ArrowSchema::empty(), we should also make FFI_ArrowArray::empty() public, which makes it easy to create empty struct and query from FFI library.

close #602

[codecov-commenter]

Codecov Report

Merging #612 (59db75a) into master (87d2840) will increase coverage by 0.01%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##           master     #612      +/-   ##
==========================================
+ Coverage   82.41%   82.42%   +0.01%     
==========================================
  Files         167      167              
  Lines       46232    46257      +25     
==========================================
+ Hits        38100    38126      +26     
+ Misses       8132     8131       -1     

Impacted Files	Coverage Δ
arrow/src/ffi.rs	87.07% <0.00%> (ø)
arrow/src/array/ord.rs	63.33% <0.00%> (ø)
arrow/src/compute/kernels/substring.rs	98.27% <0.00%> (ø)
arrow/src/array/builder.rs	86.26% <0.00%> (+0.15%)	⬆️
arrow/src/compute/kernels/partition.rs	97.65% <0.00%> (+0.15%)	⬆️
parquet/src/encodings/encoding.rs	95.04% <0.00%> (+0.19%)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 87d2840...59db75a. Read the comment docs.

[alamb]

Seems reasonable to me -- any thoughts @jorgecarleitao or @nevi-me ?

[alamb]

Thank you @wangfenjin !