Fix encode() with an empty batch and out_type='offset_mapping'

[Kayvan-Zahiri]

sp.encode([], out_type='offset_mapping') raises TypeError. Every other return type
gives [] for an empty batch.

sp.encode([], out_type=int)               # []
sp.encode([], out_type=str)               # []
sp.encode([], out_type='proto')           # []
sp.encode([], out_type='offset_mapping')  # TypeError: _EncodeAsOffsetMappingBatch():
                                          # incompatible function arguments

The default return_bytes expression is

isinstance(input, bytes) or (isinstance(input, list) and input and isinstance(input[0], bytes))

With input == [] the and chain short-circuits on the empty list, so the whole
expression evaluates to [] rather than False. That [] is appended to args and
passed to the pybind11 bool parameter, which rejects it.

Wrapping the second operand in bool() fixes it. Byte detection is unchanged: a bytes
batch still returns bytes pieces and a str batch still returns str pieces.

One regression test added, covering offset_mapping alongside int and str so the
empty-batch behavior stays consistent. It fails before and passes after.
python/test/sentencepiece_test.py goes from 38 passed to 39 with the same 6
pre-existing failures, which need a locally built extension and are unrelated.

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