[bugfix] using numeric operator in filter expression causes error

Example:

```xquery
xquery version "3.0";

let $items :=
<items>
   <item>
       <target>
           <id>uuid-538a6e13-f88b-462c-a965-f523c3e02bbf</id>
           <start>3</start>
           <offset>4</offset>
       </target>
   </item>
   <item>
       <target>
           <id>uuid-538a6e13-f88b-462c-a965-f523c3e02bbf</id>
           <start>8</start>
           <offset>15</offset>
       </target>
   </item>
</items>

let $my-item := $items/*[3 - 1]/target/start/number()
return $my-item
```