fix parsing invalid interval string

Author: rchindris

src/pendulum/parsing/__init__.py

@@ -212,6 +212,10 @@ def _parse_iso8601_interval(text: str) -> _Interval:
         raise ParserError("Invalid interval")
 
     first, last = text.split("/")
+
+    if not first or not last:
+        raise ParserError("Invalid interval.")
+
     start = end = duration = None
 
     if first[0] == "P":

tests/parsing/test_parsing_duration.py

@@ -293,6 +293,14 @@ def test_parse_duration_invalid():
         parse("P1Dasdfasdf")
 
 
+def test_parse_interval_invalid():
+    with pytest.raises(ParserError):
+        parse("/no_start")
+
+    with pytest.raises(ParserError):
+        parse("no_end/")
+
+
 def test_parse_duration_fraction_only_allowed_on_last_component():
     with pytest.raises(ParserError):
         parse("P2Y3M4DT5.5H6M7S")