Added hook for catching null value instead of empty list in path

[borysvorona]

Fixes caused by errors during parsing non-perfect JSON files with different schema.

from jsonpath_ng.ext import parse

jsonpath_expr = parse("$.store.book[*].category")

json_file = """
{
    "store": {
        "book": null,
        "bicycle": {
            "color": "red",
            "price": 19.95
        }
    },
    "expensive": 10
}
"""
result = [
    match.value for match in jsonpath_expr.find(
        json.loads(json_file)
    )
]

For example, after execution code block above got a TypeError:

  File "/usr/local/lib/python3.7/site-packages/jsonpath_ng/jsonpath.py", line 255, in find
    for subdata in self.left.find(datum)
  File "/usr/local/lib/python3.7/site-packages/jsonpath_ng/jsonpath.py", line 255, in find
    for subdata in self.left.find(datum)
  File "/usr/local/lib/python3.7/site-packages/jsonpath_ng/jsonpath.py", line 257, in <listcomp>
    for submatch in self.right.find(subdata)]
  File "/usr/local/lib/python3.7/site-packages/jsonpath_ng/jsonpath.py", line 628, in find
    return [DatumInContext(datum.value[i], path=Index(i), context=datum) for i in xrange(0, len(datum.value))]
TypeError: object of type 'NoneType' has no len()

[coveralls]

Coverage remained the same at ?% when pulling 3e36466972449bedf9ac572dfbb8115a3760d1d7 on borysvorona:master into 003a0b78f8a1857aa7027a837def54564a2b578a on h2non:master.

[borysvorona]

@h2non Could you take a look. It is a major issue. Also, I ask you to fix broken URLs in Travis CI build.