contract duplicate spaces in transliteration string

There are some pathological cases where an isolated letter may
be deleted because it is in itself meaningless. If this happens in
the middle of a sentence, then the transliteration contains two
consecutive spaces. Add a final rule to fix this.

See #2909.

settings/icu_tokenizer.yaml

@@ -24,6 +24,7 @@ transliteration:
     - ":: lower ()"
     - "[^a-z0-9[:Space:]] >"
     - ":: NFC ()"
+    - "[:Space:]+ > ' '"
 sanitizers:
     - step: clean-housenumbers
       filter-kind: