Change default behaviour of merge_staypoint()

[hongyeehh]

The function accepts sp and tpls as input. But if an empty or pd.DataFrame or gpd.Geodataframe tpls is given, this line will raise a warning as tpls has no crs.

UserWarning: CRS not set for some of the concatenation inputs. Setting output's CRS as WGS 84 (the single non-null crs provided).

In general, we do not enforce checking the correctness of sp and tpls inputs, and we explicitly mention tpls can be empty in the docstring.

It would be good to check the format of sp and separate the scenarios in the code when tpls is given as empty (maybe set input tpls=None by default).