Fix valueList without displayValue key

[LeJeko]

Just a quick fix to avoid a crash of the get_value_list_values function when a value list contains an empty entry and FileMaker does not return a displayValue key.

['valueLists':
    [
        {'name': 'my_value_list',
         'type': 'customList',
         'values':
             [ 
                {'value': ''},
                {'displayValue': 'A', 'value': 'a'},
                {'displayValue': 'B', 'value': 'b'},
                {'displayValue': 'C', 'value': 'c'}
                ]
        }
    ]
]

In this case, the function set displayValue to an empty string ''

[('', ''), ('a', 'A'), ('b', 'B'), ('c', 'C')]