Bug: Annotation starting with quotes fail upon conversion to rdf

[c0ldst0n3]

When a Literal string value starts with a quote, but does not end with a quote, the subsequent conversion to rdf fails.

The issue can be repeated by applying this piece code:

from funowl.converters.functional_converter import to_python
from rdflib import Graph

ofn = """AnnotationAssertion(rdfs:comment onto:ONTO_123 "'Some string', string continuation")"""
x = f"Ontology(<http://test/onto> {ofn})"

conv = to_python(x.encode("utf-8"))
g = Graph()
conv.to_rdf(g)

This issue happens in the Literal._to_n3 staticmethod, where only a check occurs if the string starts with a quote, but not if it ends with one. As a result, a BadSyntax error results in None to be returned.

...
# Add quotes if necessary
if len(v) == 0 or v[0] not in ['"', "'"]:
    v = '"' + v + '"'
# Create a turtle triple to use the n3 parser
# TODO: make the constructor static and use clear
stmt = f'@prefix : <{DUMMY_PREFIX}> . @prefix xsd: <{XSD}> . :f a {v} .'
g = Graph(bind_namespaces="core")
try:
    g.parse(data=stmt, format="turtle")
except BadSyntax:
    pass
...