Fix CHAR(0) to return null byte instead of empty byte array

CHAR(0) should return a single null byte (\0) not an empty byte array.
This was exposed by string-to-number conversion changes that now
correctly convert empty strings to 0 instead of erroring.

Before: CHAR('') -> error path -> append null byte
After: CHAR('') -> converts to 0 -> CHAR(0) -> should return null byte

Fixes the failing test: select if('', 1, char(''))

Author: elianddb

sql/expression/function/char.go

@@ -89,7 +89,7 @@ func (c *Char) CollationCoercibility(ctx *sql.Context) (collation sql.CollationI
 // This function is essentially converting the number to base 256
 func char(num uint32) []byte {
 	if num == 0 {
-		return []byte{}
+		return []byte{0}
 	}
 	return append(char(num>>8), byte(num&255))
 }