Bug Report: evalengine TypeOf for Columns

[systay]

While working on other parts of Vitess, we encountered this issue. The faulty code is:

func (c *Column) typeof(env *ExpressionEnv, fields []*querypb.Field) (sqltypes.Type, typeFlag) {
	// if we have an active row in the expression Env, use that as an authoritative source
	if c.Offset < len(env.Row) {
		value := env.Row[c.Offset]
		if value.IsNull() {
			return sqltypes.Null, flagNull | flagNullable
		}
		return value.Type(), typeFlag(0)
	}
// cut for brevity

The issue with this code is the NULL handling - for queries with outer joins, some rows can contain null values for a given column, and some will not. We need a better fallback for these situations.

[vmg]

We actually fixed this!