timeline and dataframe fail when system:time_end is missing

[aazuspan]

Some collections (at least Landsat 7) are missing system:time_end properties for most images, even though they have system:time_start properties. This causes dataframe to fail because the fields used to initialize the pd.Dataframe are of different lengths. I'll probably drop system:time_end from dataframe since it's rarely useful and filling missing values is not supported by aggregate_array.

Reproduce

pt = ee.Geometry.Point([-121.690476, 45.432933])

(wxee.TimeSeries("LANDSAT/LE07/C01/T1_SR")
      .filterBounds(pt).timeline()
)

Traceback

---------------------------------------------------------------------------
ValueError                                Traceback (most recent call last)
<ipython-input-53-af49cab8b808> in <module>
      1 pt = ee.Geometry.Point([-121.690476, 45.432933])
      2 
----> 3 (wxee.TimeSeries("LANDSAT/LE07/C01/T1_SR")
      4       .filterBounds(pt).timeline()
      5 )

~\anaconda3\envs\gee\lib\site-packages\wxee\time_series.py in timeline(self)
    151             A Plotly graph object interactive plot showing the acquisition time of each image in the time series.
    152         """
--> 153         df = self.dataframe()
    154         df["y"] = 0
    155 

~\anaconda3\envs\gee\lib\site-packages\wxee\time_series.py in dataframe(self)
    139         ends = [_millis_to_datetime(ms) for ms in ends_millis]
    140 
--> 141         df = pd.DataFrame({"id": ids, "time_start": starts, "time_end": ends})
    142         df.index.id = collection_id
    143         return df

~\anaconda3\envs\gee\lib\site-packages\pandas\core\frame.py in __init__(self, data, index, columns, dtype, copy)
    527 
    528         elif isinstance(data, dict):
--> 529             mgr = init_dict(data, index, columns, dtype=dtype)
    530         elif isinstance(data, ma.MaskedArray):
    531             import numpy.ma.mrecords as mrecords

~\anaconda3\envs\gee\lib\site-packages\pandas\core\internals\construction.py in init_dict(data, index, columns, dtype)
    285             arr if not is_datetime64tz_dtype(arr) else arr.copy() for arr in arrays
    286         ]
--> 287     return arrays_to_mgr(arrays, data_names, index, columns, dtype=dtype)
    288 
    289 

~\anaconda3\envs\gee\lib\site-packages\pandas\core\internals\construction.py in arrays_to_mgr(arrays, arr_names, index, columns, dtype, verify_integrity)
     78         # figure out the index, if necessary
     79         if index is None:
---> 80             index = extract_index(arrays)
     81         else:
     82             index = ensure_index(index)

~\anaconda3\envs\gee\lib\site-packages\pandas\core\internals\construction.py in extract_index(data)
    399             lengths = list(set(raw_lengths))
    400             if len(lengths) > 1:
--> 401                 raise ValueError("arrays must all be same length")
    402 
    403             if have_dicts:

ValueError: arrays must all be same length