Report error for @Nullable synchronized block expression

[msridhar]

Fixes #1103

The Caffeine JMH benchmark now fails due to the new checking; we will update in a follow-up (#1108)

[codecov]

Codecov Report

Attention: Patch coverage is 88.88889% with 1 line in your changes missing coverage. Please review.

Project coverage is 88.11%. Comparing base (d0502e8) to head (325daa3).
Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
...away/src/main/java/com/uber/nullaway/NullAway.java	88.88%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##             master    #1106      +/-   ##
============================================
+ Coverage     88.10%   88.11%   +0.01%     
- Complexity     2209     2212       +3     
============================================
  Files            84       84              
  Lines          7179     7188       +9     
  Branches       1426     1428       +2     
============================================
+ Hits           6325     6334       +9     
+ Misses          430      429       -1     
- Partials        424      425       +1     

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