Append placeholders when attibute values are not provided

[Christiaan-de-Wet]

This will append to the array so that any missing attributes does not cause the error "missing x required positional arguments"

from cpeparser import CpeParser
cpe = CpeParser()
result = cpe.parser("cpe:2.3:a:ipython:ipython")
print(result)
{
    'part': 'a',
    'vendor': 'ipython',
    'product': 'ipython',
    'version': '*',
    'update': '*',
    'edition': '*',
    'language': '*',
    'sw_edition': '*',
    'target_sw': '*',
    'target_hw': '*',
    'other': '*'
}