make lwm2m object response body pointer to avoid decode failed

[rhoninl]

What this PR does / why we need it:
if int value is 0 or empty string it will case decode error
Will this PR make the community happier?

Which issue this PR fixes (optional, in fixes #<issue number>(, fixes #<issue_number>, ...) format, will close that issue when PR gets merged): fixes #

How is this PR tested

• unit test
• e2e test
• other (please specify)
  test with leshan
  Special notes for your reviewer:

Release note:

[codecov]

Codecov Report

Attention: Patch coverage is 12.50000% with 7 lines in your changes missing coverage. Please review.

Project coverage is 32.94%. Comparing base (cfc8fd1) to head (a13bb41).
Report is 2 commits behind head on release_v0.60.0.

Files with missing lines	Patch %	Lines
pkg/gateway/lwm2m/client/object.go	12.50%	7 Missing ⚠️

Additional details and impacted files

@@                 Coverage Diff                 @@
##           release_v0.60.0    #1084      +/-   ##
===================================================
- Coverage            32.96%   32.94%   -0.03%     
===================================================
  Files                   55       55              
  Lines                 4875     4878       +3     
===================================================
  Hits                  1607     1607              
- Misses                3128     3131       +3     
  Partials               140      140              

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