BUG: Parse NULL char as null value

[gfyoung]

Fixes bug in C parser in which the NULL character ('\x00') was being interpreted as a true line terminator, escape character, or comment character because it was used to indicate that a user had not specified these values. As a result, if the data contains this value, it was being incorrectly parsed. It should be parsed as NULL.

Closes #14012.

[gfyoung]

Points of clarification to make here:

1. The reason why we still default lineterminator to '\0' instead of say '\n' is to allow for proper carriage character ('\r') detection, as can be seen here.

2. The test I added will fail if it weren't any of the modified lineterminator, escapechar, or commentchar checks. They all used to compare against '\0' if none was specified, causing that stray '\x00' character to be incorrectly parsed in some way, shape, or form.

[codecov-io]

Current coverage is 85.29% (diff: 100%)

Merging #14019 into master will not change coverage

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[jreback]

thanks!