Earley Parser handles epsilon productions incorrectly

[rasc00001]

When parsing strings that contain the result of epsilon productions with EarleyParser,
the parsed subtree containing the nullable nonterminal has an empty children list,
as if the nonterminal was a terminal.
(see description of Derivation Trees at https://www.fuzzingbook.org/html/GrammarFuzzer.html#Derivation-Trees)

To Reproduce
Steps to reproduce the behavior:
Run

from fuzzingbook.Parser import EarleyParser
next(EarleyParser({"<start>": [""]}).parse(""))

The return value is

('<start>', [])

Expected behavior
The return value is expected to be

('<start>', [('', [])])  

Additional context
This issue might cause #153