fix: removed EntityManager.clear() from GraphQLJpaQueryDataFetcher

[igdianov]

This PR removes calling EntityManager.clear() inside GraphQLJpaQueryDataFetcher.get() that causes runtime NPE in Hibernate in order to fix #191

The EntityManager.clear() has been added in this commit: 83c5a84 because of the mentioned problem: #96 (comment)

The provided test case cannot reproduce the issue anymore: 9e5002b

Calling clear method seems to be very invasive. There are already hints provided for Hibernate not to cache any results in memory:

graphql-jpa-query/graphql-jpa-query-schema/src/main/java/com/introproventures/graphql/jpa/query/schema/impl/GraphQLJpaQueryDataFetcher.java

Line 133 in 7ce947d

query.setHint(ORG_HIBERNATE_READ_ONLY, true);

Also, this problem could be still relevant in Hibernate versions before 5.3.10. The users may need to call EntityManager.clear() before execution of the query manually if needed.

[codecov]

Codecov Report

Merging #192 into master will increase coverage by 0.04%.
The diff coverage is n/a.

@@             Coverage Diff              @@
##             master     #192      +/-   ##
============================================
+ Coverage     73.63%   73.68%   +0.04%     
- Complexity      873      874       +1     
============================================
  Files            50       50              
  Lines          3702     3701       -1     
  Branches        627      627              
============================================
+ Hits           2726     2727       +1     
+ Misses          697      696       -1     
+ Partials        279      278       -1

Impacted Files	Coverage Δ	Complexity Δ
.../query/schema/impl/GraphQLJpaQueryDataFetcher.java	92.47% <ø> (-0.09%)	26 <0> (ø)
.../graphql/jpa/query/introspection/Constructors.java	82% <0%> (+4%)	15% <0%> (+1%)	⬆️

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