PROV-XML round-trip drops attributes whose value is the empty string

[trungdong]

Found by the Task 13 Hypothesis property-based round-trip tests (src/prov/tests/test_property_roundtrip.py).

An other_attributes entry whose value is the empty string "" round-trips correctly through PROV-JSON and PROV-O (RDF) but is dropped entirely by the PROV-XML serializer — the attribute vanishes, so the reloaded document is unequal to the original.

Reproduction:

import io
from prov.model import ProvDocument

def rt(d, fmt):
    s = io.BytesIO(); d.serialize(destination=s, format=fmt, indent=4); s.seek(0)
    return ProvDocument.deserialize(source=s, format=fmt)

d = ProvDocument(); d.add_namespace("ex", "http://example.org/")
d.agent("ex:g0", {"ex:k0": ""})
print("json:", rt(d, "json") == d)  # True
print("xml: ", rt(d, "xml")  == d)  # False  <-- attribute dropped
print("rdf: ", rt(d, "rdf")  == d)  # True

The XML round trip yields agent(ex:g0) with no ex:k0 attribute.

Status: deferred to 3.0.0 under the 2.x output freeze. Excluded from Hypothesis generation via min_size=1 on the text-value strategy (src/prov/tests/strategies.py), commented pointing here. No behaviour change made in 2.x.