3.7: Bluetooth: TBS: TBS client may return NULL instead of the empty string in callbacks

[Thalley]

Describe the bug
If the TBS client receives an empty value when reading a string, it treated that as NULL, but since empty strings are still strings, it should simply treat it as such, and provide "\0" instead. The callbacks are documented to return strings.

To Reproduce
Steps to reproduce the behavior:

1. Build and flash the https://github.com/zephyrproject-rtos/zephyr/tree/main/samples/bluetooth/tmap_peripheral from the 3.7 branch
2. Connect a Android 15 phone to the sample
3. See that URI list is empty when read but NULL provided to the callback

Expected behavior
Expect the TBS client to follow its documentation and always return a string when there's no read error.

Impact
Confused developers. The TMAP sample would call strlen and strncpy on a NULL value.

Logs and console output

CCP: Discovered GTBS
CCP: Discovered remote URI: �w
CCP initialized

Environment (please complete the following information):

• OS: (e.g. Linux, MacOS, Windows)
• Toolchain (e.g Zephyr SDK, ...)
• Commit SHA or Version used: 1182a4c (v3.7-branch)

Additional context
N/A

[kartben]

triaging as low due to impact being described as "confusion"