[PATCH] Change "the empty string" to "an empty string" on DOCs

[Marcos Pegoraro]

There are 64 times of "an empty string" and 13 times "the empty string"
written on DOCs, and how they are used to do the same meaning, we could
standardize them.

regards
Marcos

V1-0001 [PATCH] Change the empty string to an empty string on DOCs.diff
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