Fix calling NewTess() with an empty datapath argument

[tsudoko]

This PR makes passing an empty datapath to NewTess() actually pass NULL to TessBaseAPIInit3(). It's needed because passing NULL is the only way to make Tesseract use the compiled in TESSDATA_PREFIX constant as the datapath (and Go won't allow passing nil when the argument type is string).

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Code used to determine the datapath in Tesseract can be found in ccutil/mainblk.cpp.

[tsudoko]

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