Filter does not support empty strings

[Marc-Andre-Rivet]

Running the following code will fail with an AST error

import dash

from dash_table import DataTable

app = dash.Dash(__name__)

app.layout = DataTable(
    id='table',
    columns=[{
        'name': x,
        'id': x,
        'selectable': True
    } for x in ['a', 'b', 'c']],
    data=[{
        'a': str(x) if x % 2 == 0 else '',
        'b': x if x % 3 == 1 else '',
        'c': str(x*x) if x % 4 == 2 else ''
    } for x in range(0,100)],
    style_data_conditional=[{
        'if': {
            'column_id': x,
            'filter_query': '{{{}}} eq ""'.format(x)
        },
        'backgroundColor': 'pink'
    } for x in ['a', 'b', 'c']]
)

if __name__ == '__main__':
    app.run_server(debug=True)

This is due to https://github.com/plotly/dash-table/blob/master/src/dash-table/syntax-tree/lexeme/expression.ts#L7 requiring at least one character inside the string. Removing this requirement will make the above code work as expected.

[alexcjohnson]

Related: #544