Fix segfault on join with None (#422)

Fixes #353

Author: evertlammerts

src/duckdb_py/pyrelation.cpp

@@ -1188,6 +1188,9 @@ static JoinType ParseJoinType(const string &type) {
 
 unique_ptr<DuckDBPyRelation> DuckDBPyRelation::Join(DuckDBPyRelation *other, const py::object &condition,
                                                     const string &type) {
+	if (!other) {
+		throw InvalidInputException("No relation provided for join");
+	}
 
 	JoinType join_type;
 	string type_string = StringUtil::Lower(type);

tests/fast/api/test_join.py

@@ -45,6 +45,13 @@ def test_relational_join_with_condition(self):
         res = rel.fetchall()
         assert res == [(1, 2, 1, 3)]
 
+    def test_join_none_raises(self):
+        con = duckdb.connect()
+        rel = con.sql("SELECT 1 AS col1")
+
+        with pytest.raises(duckdb.InvalidInputException, match="No relation provided for join"):
+            rel.join(None, "col1")
+
     @pytest.mark.xfail(condition=True, reason="Selecting from a duplicate binding causes an error")
     def test_deduplicated_bindings(self, duckdb_cursor):
         duckdb_cursor.execute("create table old as select * from (values ('42', 1), ('21', 2)) t(a, b)")