Problem with tei:line when serializing B3

Extracted from f842e8e0 (comment 144194).

For some reason else if($node/following::*[1][self::tei:lb[@break = "no"]] ...) returns true() for tei:line on surface 57r which results in missing elements. The fix proposed in !66 (merged) solves this, but the reason for this unexpected behaviour is still unclear.