Return if setting NULL

[chands10]

I couldn't exploit this line not being here, seems like if inserting a NULL blob then this function doesn't get called, with cdb2sql or tagged api

Tried with

tag ondisk {
    int a
    blob b[5] null=yes
}

tag "imatag" {
    int a
}

keys {
    dup "a" = a
}

Wasn't able to try with adding a null blob field to the tag though

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