Raise a specific error when trying to transact invalid sequences.

This specifically checks for things like :db/add foo bar (nil),
which will otherwise fail elsewhere after being exploded.

src/datomish/transact/explode.cljc

@@ -60,11 +60,19 @@
                 :op    entity }))
 
       (sequential? v)
+      (if (some nil? v)
+        ;; This is a hard one to track down, with a steep stack back in `transact/ensure-entity-form`, so
+        ;; we error specifically here rather than expanding further.
+        (raise "Sequential attribute value for " a " contains nil."
+               {:error :transact/sequence-contains-nil
+                :op entity
+                :attribute a
+                :value v})
         (if (ds/multival? (db/schema db) a*) ;; dm/schema
           (mapcat (partial explode-entity-a-v db entity eid a) v) ;; Allow sequences of nested maps, etc.  This does mean [[1]] will work.
           (raise "Sequential values " v " but attribute " a " is :db.cardinality/one"
                  {:error :transact/entity-sequential-cardinality-one
-                :op    entity }))
+                  :op    entity })))
 
       true
       [[:db/add eid a* v]])))