Fix is_sorted_nb NaN parity

[mameikagou]

Summary

• make is_sorted_nb reject unordered adjacent comparisons, matching
  is_sorted when an array contains NaN
• add a regression assertion for both implementations

Previously, < returned false for a NaN pair and the Numba implementation
could incorrectly report the array as sorted.

Testing

• pytest -q tests/ (940 passed, 100 skipped)
• Black check
• python -m compileall vectorbt
• git diff --check