This patch deprecates "min_len" or "max_len" of 0 in
"byte_string_dtypes()" and "unicode_string_dtypes()". The lower limit
is now 1.
Numpy uses a length of 0 in these dtypes to indicate an undetermined
size, chosen from the data at array creation. However, as the
"arrays()" strategy creates arrays before filling them, strings were
truncated to 1 byte.
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