maven: Handle null mtime value in index for jar archive

It exists cases where the modification time for a jar archive in
a maven index is null which was leading to a processing error
by the lister.

So handle that case to avoid premature exit of the listing process.

Related to T3874

swh/lister/maven/lister.py

@@ -304,11 +304,11 @@ class MavenLister(Lister[MavenListerState, RepoPage]):
             # Origin is gathering source archives:
             last_update_dt = None
             last_update_iso = ""
-            last_update_seconds = str(page["time"])[:-3]
             try:
+                last_update_seconds = str(page["time"])[:-3]
                 last_update_dt = datetime.fromtimestamp(int(last_update_seconds))
                 last_update_dt = last_update_dt.astimezone(timezone.utc)
-            except OverflowError:
+            except (OverflowError, ValueError):
                 logger.warning("- Failed to convert datetime %s.", last_update_seconds)
             if last_update_dt:
                 last_update_iso = last_update_dt.isoformat()

swh/lister/maven/tests/data/http_indexes/export_null_mtime.fld

@@ -0,0 +1,21 @@
+doc 0
+  field 0
+    name u
+    type string
+    value al.aldi|sprova4j|0.1.0|sources|jar
+  field 1
+    name m
+    type string
+    value 1633786348254
+  field 2
+    name i
+    type string
+    value jar|0|14316|2|2|0|jar
+  field 10
+    name n
+    type string
+    value sprova4j
+  field 11
+    name d
+    type string
+    value Java client for Sprova Test Management

swh/lister/maven/tests/test_lister.py

@@ -66,6 +66,11 @@ def maven_pom_1(datadir) -> str:
     return Path(datadir, "https_maven.org", "sprova4j-0.1.0.pom").read_text()
 
 
+@pytest.fixture
+def maven_index_null_mtime(datadir) -> str:
+    return Path(datadir, "http_indexes", "export_null_mtime.fld").read_text()
+
+
 @pytest.fixture
 def maven_pom_1_malformed(datadir) -> str:
     return Path(datadir, "https_maven.org", "sprova4j-0.1.0.malformed.pom").read_text()
@@ -290,3 +295,25 @@ def test_maven_list_http_error_artifacts(
     # then we get only one maven-jar origin and one git origin.
     scheduler_origins = swh_scheduler.get_listed_origins(lister.lister_obj.id).results
     assert len(scheduler_origins) == 3
+
+
+def test_maven_lister_null_mtime(swh_scheduler, requests_mock, maven_index_null_mtime):
+
+    requests_mock.get(INDEX_URL, text=maven_index_null_mtime)
+
+    # Run the lister.
+    lister = MavenLister(
+        scheduler=swh_scheduler,
+        url=MVN_URL,
+        instance="maven.org",
+        index_url=INDEX_URL,
+        incremental=False,
+    )
+
+    stats = lister.run()
+
+    # Start test checks.
+    assert stats.pages == 1
+    scheduler_origins = swh_scheduler.get_listed_origins(lister.lister_obj.id).results
+    assert len(scheduler_origins) == 1
+    assert scheduler_origins[0].last_update is None