Prevent crash when instancing an AccountManagerServices if no URL is set in the RC

Review guidelines

Produced code must conform to:

• Coding style for liblinphone project
• Liblinphone API design guide for liblinphone public API addons or changes
• Software development good practice BC's guide

In addition, from a pure C++ usage, it is recommended to follow the C++ Core Guidelines.

Review comments should, whenever possible, be supported by references to relevant documents mentionned above. All comments and discussions between reviewers and developers must remain respectful and constructive. By grounding discussions in shared references, code reviews should help foster constructive debate and contribute to the collective adoption and dissemination of good practices across the team.

https://console.firebase.google.com/u/0/project/linphone-b3ec9/crashlytics/app/android:org.linphone/issues/8502b4a4d913e2e5725f9d8d482d575f?time=24h&versions=6.2.2%20(602002)&sessionEventKey=6a4112a801f600013d69f56f218cd8cf_2234867046728769170