Parser assumes that input is more than one line long

[obi1kenobi]

Note the following IPython session using the 1.0 version of graphql-core, where I feed the parser a simple empty string, and rather than getting a parse error, I get an unfriendly Python IndexError instead.

In [1]: from graphql.language.parser import parse

In [2]: parse('')
---------------------------------------------------------------------------
IndexError
Traceback (most recent call last)
<ipython-input> in <module>()
----> 1 parse('')

<omitted>/site-packages/graphql/language/parser.pyc in parse(source, **kwargs)
     19 
     20     parser = Parser(source_obj, options)
---> 21     return parse_document(parser)
     22 
     23 

<omitted>/site-packages/graphql/language/parser.pyc in parse_document(parser)
    188     definitions = []
    189     while True:
--> 190         definitions.append(parse_definition(parser))
    191 
    192         if skip(parser, TokenKind.EOF):

<omitted>/site-packages/graphql/language/parser.pyc in parse_definition(parser)
    213             return parse_type_system_definition(parser)
    214 
--> 215     raise unexpected(parser)
    216 
    217 

<omitted>/site-packages/graphql/language/parser.pyc in unexpected(parser, at_token)
    143         parser.source,
    144         token.start,
--> 145         u'Unexpected {}'.format(get_token_desc(token))
    146     )
    147 

<omitted>/site-packages/graphql/error/syntax_error.pyc in __init__(self, source, position, description)
     15                 location.column,
     16                 description,
---> 17                 highlight_source_at_location(source, location),
     18             ),
     19             source=source,

<omitted>/site-packages/graphql/error/syntax_error.pyc in highlight_source_at_location(source, location)
     30     if line >= 2:
     31         result += format(line - 1, lines[line - 2])
---> 32     result += format(line, lines[line - 1])
     33     result += ' ' * (1 + pad_len + location.column) + '^\n'
     34     if line < len(lines):

IndexError: list index out of range