Fix NPE when stream name is use to reference attributes in agg query

[niveathika]

Purpose

$subject, fixes #1494

Documentation

N/A

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 #1497 into master will decrease coverage by <.01%.
The diff coverage is 50%.

@@             Coverage Diff              @@
##             master    #1497      +/-   ##
============================================
- Coverage     68.15%   68.15%   -0.01%     
  Complexity       42       42              
============================================
  Files           643      643              
  Lines         26981    26984       +3     
  Branches       4260     4260              
============================================
  Hits          18390    18390              
- Misses         6805     6808       +3     
  Partials       1786     1786

Impacted Files	Coverage Δ	Complexity Δ
...core/aggregation/AggregationExpressionVisitor.java	25.68% <100%> (ø)	0 <0> (ø)	⬇️
...io/siddhi/core/aggregation/AggregationRuntime.java	52.64% <46.66%> (+1.14%)	0 <0> (ø)	⬇️
...c/main/java/io/siddhi/core/util/ThreadBarrier.java	80% <0%> (-20%)	0% <0%> (ø)
.../siddhi/core/util/event/handler/StreamHandler.java	62% <0%> (-12%)	0% <0%> (ø)
...istence/IncrementalFileSystemPersistenceStore.java	50.96% <0%> (-0.97%)	0% <0%> (ø)
...i/core/util/parser/CollectionExpressionParser.java	39.51% <0%> (+0.43%)	0% <0%> (ø)	⬇️
...java/io/siddhi/core/stream/input/InputHandler.java	80.64% <0%> (+3.22%)	0% <0%> (ø)	⬇️
.../collection/executor/AnyAndCollectionExecutor.java	29.62% <0%> (+3.7%)	0% <0%> (ø)	⬇️

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