Validate work_item_type_id is NOT NULL for new issue records

What does this MR do and why?

We need to make sure no issues are created without associating them with a work item type first. We will validate the constraint after backfilling existing records

Related to #338003 (closed)

Migration Output

UP

== 20220414161654 AddNotNullConstraintToIssuesWorkItemType: migrating =========
-- current_schema()
   -> 0.0009s
-- transaction_open?()
   -> 0.0000s
-- current_schema()
   -> 0.0003s
-- transaction_open?()
   -> 0.0000s
-- execute("ALTER TABLE issues\nADD CONSTRAINT check_2addf801cd\nCHECK ( work_item_type_id IS NOT NULL )\nNOT VALID;\n")
   -> 0.0015s
== 20220414161654 AddNotNullConstraintToIssuesWorkItemType: migrated (0.0206s)

DOWN

== 20220414161654 AddNotNullConstraintToIssuesWorkItemType: reverting =========
-- transaction_open?()
   -> 0.0000s
-- transaction_open?()
   -> 0.0000s
-- execute("ALTER TABLE issues\nDROP CONSTRAINT IF EXISTS check_2addf801cd\n")
   -> 0.0019s
== 20220414161654 AddNotNullConstraintToIssuesWorkItemType: reverted (0.0104s)

MR acceptance checklist

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