Add null check for appId getter on Number model

[erickskrauch]

Description

When the number isn't linked to the application, there are no fields voiceCallbackValue and messagesCallbackValue, so calling Number::getAppId() produces an error on access to not existing array key.

Motivation and Context

I need this functionality so I fix it.

How Has This Been Tested?

I tried it with my codebase.

Types of changes

• [x] Bug fix (non-breaking change which fixes an issue)
• [ ] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• [x] My code follows the code style of this project.
• [ ] My change requires a change to the documentation.
• [ ] I have updated the documentation accordingly.
• [ ] I have read the CONTRIBUTING document.
• [ ] I have added tests to cover my changes.
• [x] All new and existing tests passed.

[codecov-commenter]

Codecov Report

Merging #443 (5f63467) into main (9ebd564) will increase coverage by 0.04%. Report is 1 commits behind head on main. The diff coverage is 89.47%.

@@             Coverage Diff              @@
##               main     #443      +/-   ##
============================================
+ Coverage     78.87%   78.92%   +0.04%     
  Complexity     2324     2324              
============================================
  Files           218      218              
  Lines          6253     6248       -5     
============================================
- Hits           4932     4931       -1     
+ Misses         1321     1317       -4     

Files	Coverage Δ
src/Client/Credentials/Keypair.php	91.17% <94.44%> (+9.12%)	:arrow_up:
src/Numbers/Number.php	70.00% <0.00%> (ø)

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[SecondeJK]

LGTM, thank you for your contribution!