fix: getall method in Headers datastructure

[sobolevn]

Refs: #4927

Changes:

• KeyError now always has the key which is missing, like the default Python does:

>>> {}[1]
Traceback (most recent call last):
  File "<python-input-0>", line 1, in <module>
    {}[1]
    ~~^^^
KeyError: 1

• Empty list is not treated as the valid default

---

📚 Documentation preview 📚: https://litestar-org.github.io/litestar-docs-preview/4932

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 67.30%. Comparing base (143fae3) to head (7b80173).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #4932   +/-   ##
=======================================
  Coverage   67.30%   67.30%           
=======================================
  Files         293      293           
  Lines       15230    15230           
  Branches     1728     1728           
=======================================
  Hits        10251    10251           
  Misses       4832     4832           
  Partials      147      147           

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

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.
• 📦 JS Bundle Analysis: Save yourself from yourself by tracking and limiting bundle sizes in JS merges.