database/sql: check if src is nil before converting to string

database/sql: check if src is nil before converting to string

A nil src (NULL database value) will result in a "nil" string,
which will never parse correctly in a ParseInt or similar
numeric conversion. The resulting error is confusing. Check
for a nil src prior to converting the value to string
if the resulting string will be parsed after that.

Closes #31274

Change-Id: I90f12cceff00fbbfdd3e343b04fa7e2596390e6d
Reviewed-on: https://go-review.googlesource.com/c/go/+/174177
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>