fix(clause): clause equal empty array

[whcao]

• Do only one thing
• Non breaking API changes
• Tested

What did this pull request do?

when the value of clause.Eq is an empty array, the SQL should be IN (NULL)

User Case Description

CASE: clause.Eq{Column: column, Value: []string{}}
ERROR: syntax error at or near ")" (SQLSTATE 42601)

[jinzhu]

Hi @whcao

Thank you for your PR, NOT IN (NULL) will match nothing, so we want to raise this case as an error to avoid misunderstanding.

[jinzhu]

Maybe we can only change the Eq expression, which looks good to me and it don't cause the misunderstanding.

Would you mind to create another pull request?