[release/8.0.1xx-xcode15.4] Fix described in #21132

[vs-mobiletools-engineering-service2]

Before PR
NSUrlSessionHandler.Proxy property throws NSE when client code set any value (including null)

After PR
NSUrlSessionHandler.Proxy property throws NSE only when the client code tries to set a value different from null (Custom proxy is not supported right now in iOS implementation so setting a value other than null must throws NSE)

The reason behind the change is described in #21132

PR originally submitted by @adospace in #21137.

Backport of #21140

[vs-mobiletools-engineering-service2]

💻 [CI Build] Tests on macOS X64 - Mac Sonoma (14) passed 💻

✅ All tests on macOS X64 - Mac Sonoma (14) passed.

Pipeline on Agent
Hash: cd5130b9923ef54dd97d6728c44da72685136d0b [PR build]

[vs-mobiletools-engineering-service2]

💻 [CI Build] Tests on macOS M1 - Mac Big Sur (11) passed 💻

✅ All tests on macOS M1 - Mac Big Sur (11) passed.

Pipeline on Agent
Hash: cd5130b9923ef54dd97d6728c44da72685136d0b [PR build]

[vs-mobiletools-engineering-service2]

💻 [CI Build] Tests on macOS M1 - Mac Ventura (13) passed 💻

✅ All tests on macOS M1 - Mac Ventura (13) passed.

Pipeline on Agent
Hash: cd5130b9923ef54dd97d6728c44da72685136d0b [PR build]

[vs-mobiletools-engineering-service2]

💻 [CI Build] Tests on macOS M1 - Mac Monterey (12) passed 💻

✅ All tests on macOS M1 - Mac Monterey (12) passed.

Pipeline on Agent
Hash: cd5130b9923ef54dd97d6728c44da72685136d0b [PR build]

[vs-mobiletools-engineering-service2]

❌ [CI Build] Windows Integration Tests failed ❌

❌ Failed ❌

Pipeline on Agent
Hash: cd5130b9923ef54dd97d6728c44da72685136d0b [PR build]

[vs-mobiletools-engineering-service2]

💻 [CI Build] Windows Integration Tests passed 💻

✅ All Windows Integration Tests passed.

Pipeline on Agent
Hash: cd5130b9923ef54dd97d6728c44da72685136d0b [PR build]

[vs-mobiletools-engineering-service2]

🚀 [CI Build] Test results 🚀

Test results

✅ All tests passed on VSTS: test results.

🎉 All 98 tests passed 🎉

Tests counts

✅ cecil: All 1 tests passed. Html Report (VSDrops) Download
✅ dotnettests (iOS): All 1 tests passed. Html Report (VSDrops) Download
✅ dotnettests (MacCatalyst): All 1 tests passed. Html Report (VSDrops) Download
✅ dotnettests (macOS): All 1 tests passed. Html Report (VSDrops) Download
✅ dotnettests (Multiple platforms): All 1 tests passed. Html Report (VSDrops) Download
✅ dotnettests (tvOS): All 1 tests passed. Html Report (VSDrops) Download
✅ framework: All 2 tests passed. Html Report (VSDrops) Download
✅ fsharp: All 4 tests passed. Html Report (VSDrops) Download
✅ generator: All 1 tests passed. [attempt 3] Html Report (VSDrops) Download
✅ install-source: All 1 tests passed. Html Report (VSDrops) Download
✅ interdependent-binding-projects: All 4 tests passed. [attempt 2] Html Report (VSDrops) Download
✅ introspection: All 4 tests passed. [attempt 2] Html Report (VSDrops) Download
✅ linker: All 40 tests passed. [attempt 2] Html Report (VSDrops) Download
⚠️ mac-binding-project: No tests selected. Html Report (VSDrops) Download
⚠️ mmp: No tests selected. Html Report (VSDrops) Download
⚠️ mononative: No tests selected. Html Report (VSDrops) Download
✅ monotouch (iOS): All 7 tests passed. [attempt 3] Html Report (VSDrops) Download
✅ monotouch (MacCatalyst): All 7 tests passed. [attempt 3] Html Report (VSDrops) Download
✅ monotouch (macOS): All 8 tests passed. [attempt 3] Html Report (VSDrops) Download
✅ monotouch (tvOS): All 7 tests passed. [attempt 2] Html Report (VSDrops) Download
✅ msbuild: All 2 tests passed. [attempt 2] Html Report (VSDrops) Download
⚠️ mtouch: No tests selected. [attempt 2] Html Report (VSDrops) Download
⚠️ xammac: No tests selected. Html Report (VSDrops) Download
✅ xcframework: All 4 tests passed. [attempt 2] Html Report (VSDrops) Download
✅ xtro: All 1 tests passed. Html Report (VSDrops) Download

Pipeline on Agent
Hash: cd5130b9923ef54dd97d6728c44da72685136d0b [PR build]