fix: Do not panic in strptime() if format ends with '%'

[etiennebacher]

import polars as pl
df=pl.DataFrame({"a": ["2020-01-01"]})
df.select(pl.col("a").str.strptime(pl.Date, format="d%"))

Before:

pyo3_runtime.PanicException: invalid pattern

After:

polars.exceptions.InvalidOperationError: conversion from `str` to `date` failed in column 'a' for 1 out of 1 values: ["2020-01-01"]

You might want to try:
- setting `strict=False` to set values that cannot be converted to `null`
- using `str.strptime`, `str.to_date`, or `str.to_datetime` and providing a format string

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 79.82%. Comparing base (d15bfe5) to head (0d5c34c).
Report is 6 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #21176      +/-   ##
==========================================
- Coverage   79.88%   79.82%   -0.07%     
==========================================
  Files        1593     1593              
  Lines      227640   227642       +2     
  Branches     2600     2600              
==========================================
- Hits       181856   181708     -148     
- Misses      45187    45337     +150     
  Partials      597      597              

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.