Fix an issue that nan check generates error

[peterhys]

Description

Fix an issue that real_dpol_sat_td generates error when 3D array with nan values is applied.

Changes Made

• Change nan value checking of real_dpol_sat_td

Type of Pull Request

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation update
• Code refactoring
• Performance improvement
• Test coverage improvement
• Other (please describe):

Testing

• All existing tests pass (tox)
• Added new tests for new functionality
• Manual testing performed (describe below)

Documentation

• Updated docstrings for new/modified functions and classes
• Updated relevant documentation files (if applicable)
• Added usage examples (if applicable)
• Documentation builds without errors (cd docs && make html)

Checklist

• Code follows PEP 8 style guidelines (use Black formatter's default settings)
• Used meaningful variable and function names
• Code is modular and readable
• Commented code, particularly in hard-to-understand areas
• Changes generate no new warnings or errors
• PR focuses on a single feature or bug fix