Fix adding users to a blank database

lib/Database.php

@@ -271,7 +271,7 @@ class Database {
         }
         $hash = (strlen($password) > 0) ? password_hash($password, \PASSWORD_DEFAULT) : "";
         // NOTE: This roundabout construction (with 'select' rather than 'values') is required by MySQL, because MySQL is riddled with pitfalls and exceptions
-        $this->db->prepare("INSERT INTO arsse_users(id,password,num) select ?, ?, ((select max(num) from arsse_users) + 1)", "str", "str")->runArray([$user,$hash]);
+        $this->db->prepare("INSERT INTO arsse_users(id,password,num) select ?, ?, (coalesce((select max(num) from arsse_users), 0) + 1)", "str", "str")->runArray([$user,$hash]);
         return true;
     }
 

tests/cases/Database/SeriesUser.php

@@ -205,4 +205,11 @@ trait SeriesUser {
         $this->assertException("alreadyExists", "User", "ExceptionConflict");
         Arsse::$db->userRename("john.doe@example.com", "jane.doe@example.com");
     }
+
+    public function testAddFirstUser(): void {
+        // first truncate the users table
+        static::$drv->exec("DELETE FROM arsse_users");
+        // add a user; if the max of the num column is not properly coalesced, this will result in a constraint violation
+        $this->assertTrue(Arsse::$db->userAdd("john.doe@example.com", ""));
+    }
 }