Regression in 42.0.0: Parsing fractional intervals without leading 0 is not supported

[alamb]

Describe the bug
in version 41.0.0 an interval such as .5 months was parsed correctly

In version 42.0.0 (not yet released at the time of this writing) it fails with an error: NotYetImplemented("Unsupported Interval Expression with value ".5 months")

To Reproduce
Try to parse .5

Here is a unit test:

diff --git a/arrow-cast/src/parse.rs b/arrow-cast/src/parse.rs
index accce99b4..7984aeb9a 100644
--- a/arrow-cast/src/parse.rs
+++ b/arrow-cast/src/parse.rs
@@ -1798,6 +1798,16 @@ mod tests {
             Interval::parse("-1.5 months -3.2 days", &config).unwrap(),
         );
 
+        assert_eq!(
+            Interval::new(0i32, 15i32, 0),
+            Interval::parse("0.5 months", &config).unwrap(),
+        );
+
+        assert_eq!(
+            Interval::new(0i32, 15i32, 0),
+            Interval::parse(".5 months", &config).unwrap(),
+        );
+
         assert_eq!(
             Interval::new(2i32, 10i32, 9 * NANOS_PER_HOUR),
             Interval::parse("2.1 months 7.25 days 3 hours", &config).unwrap(),

The second case fails like this:

---- parse::tests::test_parse_interval stdout ----
thread 'parse::tests::test_parse_interval' panicked at 'called `Result::unwrap()` on an `Err` value: NotYetImplemented("Unsupported Interval Expression with value \".5 months\"")', arrow-cast/src/parse.rs:1808:51
stack backtrace:

Expected behavior
Both new cases should pass correctly

Additional context
Found while testing this upgrade in DataFusion

[alamb]

I believe this was introduced by #4291

at ac9c6fa, these two (equivalent tests) pass:

        assert_eq!(
            (0i32, 15i32, 0),
            parse_interval("months", "0.5 months").unwrap(),
        );

        assert_eq!(
            (0i32, 15i32, 0),
            parse_interval("months", ".5 months").unwrap(),
        );
 

at 72f84b2 , (with #4291), the reproducer fails:

thread 'parse::tests::test_parse_interval' panicked at 'called `Result::unwrap()` on an `Err` value: NotYetImplemented("Unsupported Interval Expression with value \".5 months\"")', arrow-cast/src/parse.rs:1808:51
stack backtrace:

[alamb]

For better (likely for worse) .5 is supported by postgres:

postgres=# select interval '0.5 minutes';
 interval
----------
 00:00:30
(1 row)

postgres=# select interval '.5 minutes';
 interval
----------
 00:00:30
(1 row)