Prevent NPE with empty Spring JPA specifications

[murdos]

An empty specification can be built with Specification.where(null) and will result in a null predicate, causing a null pointer exception:

00:05:51.901 [io-executor-thread-1] ERROR i.m.h.s.netty.RoutingInBoundHandler - Unexpected error occurred: null
java.lang.NullPointerException: null
	at org.hibernate.query.criteria.internal.predicate.BooleanExpressionPredicate.render(BooleanExpressionPredicate.java:48)
	at org.hibernate.query.criteria.internal.predicate.AbstractSimplePredicate.render(AbstractSimplePredicate.java:48)
	at org.hibernate.query.criteria.internal.QueryStructure.renderWhereClause(QueryStructure.java:351)
	at org.hibernate.query.criteria.internal.QueryStructure.render(QueryStructure.java:238)
	at org.hibernate.query.criteria.internal.CriteriaQueryImpl.interpret(CriteriaQueryImpl.java:297)
	at org.hibernate.query.criteria.internal.compile.CriteriaCompiler.compile(CriteriaCompiler.java:165)
	at org.hibernate.internal.AbstractSharedSessionContract.createQuery(AbstractSharedSessionContract.java:742)
	at org.hibernate.internal.AbstractSessionImpl.createQuery(AbstractSessionImpl.java:23)
	at io.micronaut.data.spring.jpa.intercept.FindAllSpecificationInterceptor.intercept(FindAllSpecificationInterceptor.java:74)
	at io.micronaut.data.intercept.DataIntroductionAdvice.intercept(DataIntroductionAdvice.java:80)
	at io.micronaut.aop.chain.MethodInterceptorChain.proceed(MethodInterceptorChain.java:82)
	at io.micronaut.validation.ValidatingInterceptor.intercept(ValidatingInterceptor.java:139)
	at io.micronaut.aop.chain.MethodInterceptorChain.proceed(MethodInterceptorChain.java:82)

[graemerocher]

Thanks for the contribution!