filter_query of style_**_conditional cannot find empty value

[laoxu0212]

Here are the background and the requirement:

For each row of my data, some points are missing, which will leave some cells blank.
e.g.
columns=[{'name': 'a', 'id': 'a'}, {'name': 'b', 'id': 'b'}, {'name': 'c', 'id': 'c'}]
data=[{'a':1, 'b':2}, {'b':1, 'c':2},{'a':2, 'c':3}]

I would like to highlight some missing data by changing its cell's background color.

Therefore, I should use

style_data_conditional = [{
'if':{
'column_id': 'c',
'filter_query': '{c} = ????'}},
'backgroundColor': "#3D9970"
]

to filter the empty cell.

But I don't know what to write for ???? in code. I did not find it in the documents neither.

I know I could modify my data like data[0]['c'] = 'N/A' and filter that out. But it seems that the Operators from https://dash.plot.ly/datatable/filtering can only find valid data, is there or will there be anyway to filter out those empty cell without making my data redundant?

Thanks!