ARROW-5419: [C++] Allow recognizing empty strings as null strings in CSV files

Author: Antoine Pitrou <[email protected]>

Closes #4396 from pitrou/ARROW-5419-csv-empty-null-strings and squashes the following commits:

cb086d1 <Antoine Pitrou> ARROW-5419:  Allow recognizing empty strings as null strings in CSV files

Author: pitrou

cpp/src/arrow/csv/converter-test.cc

@@ -130,7 +130,7 @@ TEST(BinaryConversion, Nulls) {
   options.strings_can_be_null = true;
   AssertConversion<BinaryType, std::string>(binary(), {"ab,N/A\n", "NULL,\n"},
                                             {{"ab", ""}, {"", ""}},
-                                            {{true, false}, {false, true}}, options);
+                                            {{true, false}, {false, false}}, options);
 }
 
 TEST(StringConversion, Basics) {
@@ -152,7 +152,7 @@ TEST(StringConversion, Nulls) {
   options.strings_can_be_null = true;
   AssertConversion<StringType, std::string>(utf8(), {"ab,N/A\n", "NULL,\n"},
                                             {{"ab", ""}, {"", ""}},
-                                            {{true, false}, {false, true}}, options);
+                                            {{true, false}, {false, false}}, options);
 }
 
 TEST(StringConversion, Errors) {

cpp/src/arrow/csv/converter.cc

@@ -144,7 +144,7 @@ class VarSizeBinaryConverter : public ConcreteConverter {
 
     if (options_.strings_can_be_null) {
       auto visit = [&](const uint8_t* data, uint32_t size, bool quoted) -> Status {
-        if (size > 0 && IsNull(data, size, false /* quoted */)) {
+        if (IsNull(data, size, false /* quoted */)) {
           builder.UnsafeAppendNull();
           return Status::OK();
         } else {