Adapt dataframe concatenation to avoid FutureWarning from Pandas

[pirmink]

• pytrajplot version: 1.1.2
• Python version: 3.11

Description

Runtime warning from pandas:
.../miniconda3/envs/pytrajplot_v1.1.2/lib/python3.11/site-packages/pytrajplot/generate_pdf.py:599: FutureWarning: The behavior of array concatenation with empty entries is deprecated. In a future version, this will no longer exclude empty items when determining the result dtype. To retain the old behavior, exclude the empty entries before the concat operation.
trajectory_longitude_expansion = pd.concat(

What I Did

PWD=/users/kaufmann/scratch/lagranto/job/large_area_bug
pytrajplot . . --datatype png --datatype pdf --domain dynamic --domain dynamic_zoom