traces: Do not print unknown location

Solves 1/3 of the infinite recursion at unknown location meme.
See #8879 for ensuring we always have a trace (for stack overflows)
We might want to re-add this for finding missing location info
*while hacking on that problem only*.

src/libutil/error.hh

@@ -70,6 +70,13 @@ struct AbstractPos
     uint32_t line = 0;
     uint32_t column = 0;
 
+    /**
+     * An AbstractPos may be a "null object", representing an unknown position.
+     *
+     * Return true if this position is known.
+     */
+    inline operator bool() const { return line != 0; };
+
     /**
      * Return the contents of the source file.
      */