Describe the bug

filter_words in pyaptamer/utils/_base.py silently emits a RuntimeWarning
when called with an empty dictionary, instead of raising a clear ValueError.

This is dangerous because filter_words is called directly in production code
at pyaptamer/aptatrans/_pipeline.py line 143:

prot_words = filter_words(prot_words)

If a user passes an empty prot_words dict to the AptaTrans pipeline, execution
silently continues with an empty vocabulary — no crash at the point of bad input,
leading to mysterious failures downstream.

To Reproduce

from pyaptamer.utils._base import filter_words

filter_words({})
# RuntimeWarning: Mean of empty slice
# RuntimeWarning: invalid value encountered in scalar divide
# silently returns {} — no error raised

Expected behavior

An empty words dict should raise a ValueError with a clear message,
consistent with how other functions in the codebase handle invalid inputs
(e.g., rna2vec, MCTS, PSeAAC all raise ValueError for bad inputs).

filter_words({})
# ValueError: `words` must not be empty.

Additional context

• Affected file: pyaptamer/utils/_base.py
• Also called in: pyaptamer/aptatrans/_pipeline.py line 143
• Fix is straightforward: add an empty check + Raises section to docstring
• A test test_filter_words_empty_dict should be added to
  pyaptamer/utils/tests/test_base.py