fix(java-emitter): check for null callback

[swaroopjagadish]

Null callbacks were not being checked before invocation.

Checklist

• The PR conforms to DataHub's Contributing Guideline (particularly Commit Message Format)
• Links to related issues (if applicable)
• Tests for the changes have been added/updated (if applicable)
• Docs related to the changes have been added/updated (if applicable)

[github-actions]

Unit Test Results

     48 files  ±0       48 suites  ±0   47m 46s ⏱️ -54s
   676 tests ±0     624 ✔️ ±0  52 💤 ±0  0 ❌ ±0 
1 504 runs  +2  1 426 ✔️ +8  78 💤  - 6  0 ❌ ±0 

Results for commit 685fab2. ± Comparison against base commit 9a237e9.

♻️ This comment has been updated with latest results.

[shirshanka]

LGTM!