Handle empty array in the subscript operator

Addresses #2133 
can't really return ``void``.

```
Caused by: java.lang.UnsupportedOperationException: not yet implemented: void
	at com.facebook.presto.sql.gen.ByteCodeUtils.unboxPrimitive(ByteCodeUtils.java:132)
	at com.facebook.presto.sql.gen.ByteCodeUtils.generateInvocation(ByteCodeUtils.java:194)
	at com.facebook.presto.sql.gen.ByteCodeGeneratorContext.generateCall(ByteCodeGeneratorContext.java:77)
	at com.facebook.presto.sql.gen.FunctionCallCodeGenerator.generateExpression(FunctionCallCodeGenerator.java:49)
	at com.facebook.presto.sql.gen.ByteCodeExpressionVisitor.visitCall(ByteCodeExpressionVisitor.java:114)
	at com.facebook.presto.sql.gen.ByteCodeExpressionVisitor.visitCall(ByteCodeExpressionVisitor.java:41)
	at com.facebook.presto.sql.relational.CallExpression.accept(CallExpression.java:87)
	at com.facebook.presto.sql.gen.PageProcessorCompiler.generateProjectMethod(PageProcessorCompiler.java:267)
	at com.facebook.presto.sql.gen.PageProcessorCompiler.generateMethods(PageProcessorCompiler.java:77)
	at com.facebook.presto.sql.gen.ExpressionCompiler.compileProcessor(ExpressionCompiler.java:119)
	at com.facebook.presto.sql.gen.ExpressionCompiler.compileAndInstantiate(ExpressionCompiler.java:95)
	at com.facebook.presto.sql.gen.ExpressionCompiler.access$300(ExpressionCompiler.java:44)
	at com.facebook.presto.sql.gen.ExpressionCompiler$1.load(ExpressionCompiler.java:55)
	at com.facebook.presto.sql.gen.ExpressionCompiler$1.load(ExpressionCompiler.java:50)
```