Fix NPE when count() AttributeFunction is used

[tishan89]

Purpose

Fix wso2/product-sp#1039

Security checks

• Followed secure coding standards in http://wso2.com/technical-reports/wso2-secure-engineering-guidelines? yes
• Ran FindSecurityBugs plugin and verified report? yes
• Confirmed that this PR doesn't commit any keys, passwords, tokens, usernames, or other secrets? yes

[codecov-io]

Codecov Report

Merging #1485 into master will decrease coverage by 0.02%.
The diff coverage is 80%.

@@             Coverage Diff              @@
##             master    #1485      +/-   ##
============================================
- Coverage     68.13%   68.11%   -0.03%     
  Complexity       42       42              
============================================
  Files           643      643              
  Lines         26967    26968       +1     
  Branches       4256     4257       +1     
============================================
- Hits          18375    18370       -5     
- Misses         6803     6806       +3     
- Partials       1789     1792       +3

Impacted Files	Coverage Δ	Complexity Δ
...io/siddhi/core/table/record/ExpressionBuilder.java	56.47% <80%> (-0.23%)	0 <0> (ø)
...c/main/java/io/siddhi/core/util/ThreadBarrier.java	80% <0%> (-20%)	0% <0%> (ø)
...istence/IncrementalFileSystemPersistenceStore.java	50.96% <0%> (-1.93%)	0% <0%> (ø)

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