Add Query system tests, fix value mapping bugs (#520)

Author: jdpedrie

Database.php

@@ -1086,6 +1086,35 @@ public function delete($table, KeySet $keySet, array $options = [])
      * $transaction = $result->transaction();
      * ```
      *
+     * ```
+     * // Parameters which may be null must include an expected parameter type.
+     * $result = $database->execute('SELECT * FROM Posts WHERE lastModifiedTime = @timestamp', [
+     *     'parameters' => [
+     *         'timestamp' => $timestamp
+     *     ],
+     *     'types' => [
+     *         'timestamp' => ValueMapper::TYPE_TIMESTAMP
+     *     ]
+     * ]);
+     *
+     * $neverEditedPosts = $result->rows();
+     * ```
+     *
+     * ```
+     * // Array parameters which may be null or empty must include the array value type.
+     * $result = $database->execute('SELECT @emptyArrayOfIntegers as numbers', [
+     *     'parameters' => [
+     *         'emptyArrayOfIntegers' => []
+     *     ],
+     *     'types' => [
+     *         'emptyArrayOfIntegers' => [ValueMapper::TYPE_ARRAY, ValueMapper::TYPE_INT64]
+     *     ]
+     * ]);
+     *
+     * $row = $result->rows()->current();
+     * $emptyArray = $row['numbers'];
+     * ```
+     *
      * @codingStandardsIgnoreStart
      * @see https://cloud.google.com/spanner/reference/rpc/google.spanner.v1#google.spanner.v1.ExecuteSqlRequest ExecuteSqlRequest
      * @codingStandardsIgnoreEnd

ValueMapper.php

@@ -242,7 +242,11 @@ private function decodeValue($value, array $type)
                 break;
 
             case self::TYPE_STRUCT:
-                $value = $this->decodeValues($type['structType']['fields'], $value, Result::RETURN_ASSOCIATIVE);
+                $fields = isset($type['structType']['fields'])
+                    ? $type['structType']['fields']
+                    : [];
+
+                $value = $this->decodeValues($fields, $value, Result::RETURN_ASSOCIATIVE);
                 break;
 
             case self::TYPE_FLOAT64:
@@ -354,7 +358,7 @@ private function paramType($value, $givenType = null, $arrayType = null)
 
                 $type = $this->typeObject(
                     self::TYPE_ARRAY,
-                    $this->typeObject((isset($types[0])) ? $types[0] : null),
+                    $this->typeObject((isset($types[0])) ? $types[0] : $arrayType),
                     'arrayElementType'
                 );