NULL values don't work correctly when implementing LOGICAL_AND aggregation function in ZetaSQL

Reference: https://github.com/google/zetasql/blob/master/docs/functions-and-operators.md#logical_and 

Problems: 

After implementation, there is a problem:

• When input contains all null values:

SELECT LOGICAL_AND(a) AS logical_and FROM UNNEST([null, null, null]) AS a;

the output is:

No matching signature for aggregate function LOGICAL_AND for argument types: INT64. Supported signature: LOGICAL_AND(BOOL)  

         which is same behavior as of BigQuery console. Though, it should return null.

How to test: unit tests