Revert "IndexedString::toUrl: Fix too-strict assert"

This assert only triggered during a valgrind run executing the
duchainify tool.

str() returned an empty string in my case, however the IndexedString
ctors ensure that isEmpty() would be true whenever empty strings were passed
initially. Thus we should return early in the toUrl function, and never trigger the assert.

There must be a different issue, I'll try to reproduce the crash and check...

CCMAIL: mail@milianw.de

This reverts commit 221534385f0c1e51cfed762cdd9a515260ba12a7.