fix invisible null constraints in sql_table

[landmaj]

Thix PR fixes issue #1655

Diagram

table: {
    shape: sql_table
    a: int {constraint: null}
    b: int {constraint: [TRUE; False; null; NULL]}
}

Before

After

Summary

The behavior of null, when used in arrays, is now consistent with how other types work. When used as the only constraint (without array), only types which implement d2ast.String are allowed. Null is also allowed but does nothing, as expected. In arrays all types are allowed but booleans are converted to lower case. Null was previously converted to an empty string, this PR changes it to lower case string "null".

This could also be fixed by changing return value of ScalarString() method for Null type. I tried it and all tests pass, however there is this commit: 9d0c73c

Another solution would be to disallow everything but d2ast.String in constraint arrays, to keep it consistent with how non-array constraints work, however this could break existing diagrams and prevent them from compiling. Let me know if you would prefer this solution.

[landmaj]

BTW your new pipeline rejects SSH signatures on commits. Is this intended and I need to figure out GPG?

[gavin-ts]

BTW your new pipeline rejects SSH signatures on commits. Is this intended and I need to figure out GPG?

this is not intended looking into it

Update: this should be fixed after rebasing on master with #1661

[alixander]

perfect PR. thank you @landmaj .