Crash fix when mac is NULL and add retry with fallback mac

[sadhyama]

RDKB-62439 Webconfig crash observed when device mac is NULL

[codecov]

Codecov Report

❌ Patch coverage is 81.35593% with 11 lines in your changes missing coverage. Please review.
✅ Project coverage is 82.97%. Comparing base (0281536) to head (4fae64b).
⚠️ Report is 2 commits behind head on master.

Files with missing lines	Patch %	Lines
src/webcfg_multipart.c	66.66%	4 Missing and 7 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #256      +/-   ##
==========================================
+ Coverage   82.94%   82.97%   +0.03%     
==========================================
  Files          38       38              
  Lines       11748    11786      +38     
  Branches     1627     1631       +4     
==========================================
+ Hits         9744     9780      +36     
+ Misses        972      968       -4     
- Partials     1032     1038       +6     

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