ARROW-75: Fix handling of empty strings

Fixes [ARROW-75](https://issues.apache.org/jira/browse/ARROW-75) (and changes Python tests to verify that behavior).

Author: Dan Robinson <danrobinson010@gmail.com>

Closes #32 from danrobinson/ARROW-75 and squashes the following commits:

cb8e527 [Dan Robinson] ARROW-75: remove whitespace
9604a21 [Dan Robinson] ARROW-75: Changed tests
722df19 [Dan Robinson] ARROW-75: Fixed braces
1ef3b75 [Dan Robinson] ARROW-75: Fix handling of empty strings

cpp/src/arrow/types/primitive.h

@@ -168,7 +168,9 @@ class PrimitiveBuilder : public ArrayBuilder {
  int32_t new_capacity = util::next_power2(length_ + length);
  RETURN_NOT_OK(Resize(new_capacity));
  }
- memcpy(raw_buffer() + length_, values, length * elsize_);
+ if (length > 0) {
+ memcpy(raw_buffer() + length_, values, length * elsize_);
+ }
 
  if (null_bytes != nullptr) {
  AppendNulls(null_bytes, length);

cpp/src/arrow/types/string-test.cc

@@ -181,7 +181,7 @@ class TestStringBuilder : public TestBuilder {
 };
 
 TEST_F(TestStringBuilder, TestScalarAppend) {
- std::vector<std::string> strings = {"a", "bb", "", "", "ccc"};
+ std::vector<std::string> strings = {"", "bb", "a", "", "ccc"};
  std::vector<uint8_t> is_null = {0, 0, 0, 1, 0};
 
  int N = strings.size();

python/pyarrow/tests/test_array.py

@@ -39,13 +39,13 @@ def test_list_format(self):
  assert result == expected
 
  def test_string_format(self):
- arr = pyarrow.from_pylist(['foo', None, 'bar'])
+ arr = pyarrow.from_pylist(['', None, 'foo'])
  result = fmt.array_format(arr)
  expected = """\
 [
- 'foo',
+ '',
  NA,
- 'bar'
+ 'foo'
 ]"""
  assert result == expected