Fix earliest UID assertion: Closes #7390

MIN() and MAX() in Sqlite returns an unanticipated NULL when a column
result set is empty.  Fix is to catch when that happens.

src/engine/db/db-result.vala

@@ -43,6 +43,18 @@ public class Geary.Db.Result : Geary.Db.Context {
         return !finished;
     }
     
+    /**
+     * column is zero-based.
+     */
+    public bool is_null_at(int column) throws DatabaseError {
+        verify_at(column);
+        
+        bool is_null = statement.stmt.column_type(column) == Sqlite.NULL;
+        log("is_null_at(%d) -> %s", column, is_null.to_string());
+        
+        return is_null;
+    }
+    
     /**
      * column is zero-based.
      */
@@ -148,6 +160,14 @@ public class Geary.Db.Result : Geary.Db.Context {
             throw new DatabaseError.LIMITS("column %d >= %d", column, count);
     }
     
+    /**
+     * name is the name of the column in the result set.  See Statement.get_column_index() for name
+     * matching rules.
+     */
+    public bool is_null_for(string name) throws DatabaseError {
+        return is_null_at(convert_for(name));
+    }
+    
     /**
      * name is the name of the column in the result set.  See Statement.get_column_index() for name
      * matching rules.

src/engine/imap-db/imap-db-folder.vala

@@ -699,7 +699,8 @@ private class Geary.ImapDB.Folder : BaseObject, Geary.ReferenceSemantics {
             stmt.bind_rowid(0, folder_id);
             
             Db.Result results = stmt.exec(cancellable);
-            if (!results.finished)
+            // MIN and MAX return NULL if the result set being examined is zero-length
+            if (!results.finished && !results.is_null_at(0))
                 id = new ImapDB.EmailIdentifier(results.rowid_at(1), new Imap.UID(results.int64_at(0)));
             
             return Db.TransactionOutcome.DONE;