fix bug in `Dokument.sortiert` mit `nummer` ohne Ziffern.

Dies verhinderte das Darstellen eines Versanddatums.
Die folgende Exception wurde im Errbit geloggt:

```
ActiveRecord::StatementInvalid: PG::InvalidTextRepresentation: FEHLER: ungültige Eingabesyntax für ganze Zahl: „“ : SELECT "dokumente"."id" AS t0_r0, "dokumente"."nummer" AS t0_r1, "dokumente"."laufnummer" AS t0_r2, "dokumente"."name" AS t0_r3, "dokumente"."source" AS t0_r4, "dokumente"."created_at" AS t0_r5, "dokumente"."updated_at" AS t0_r6, "dokumente"."versanddatum_id" AS t0_r7, "dokumente"."journal_eintrag_id" AS t0_r8, "dokumente"."cache_publikation" AS t0_r9, "journal_eintraege"."id" AS t1_r0, "journal_eintraege"."geschaeft_id" AS t1_r1, "journal_eintraege"."erfasser_id" AS t1_r2, "journal_eintraege"."datum" AS t1_r3, "journal_eintraege"."stand_ref" AS t1_r4, "journal_eintraege"."bemerkung" AS t1_r5, "journal_eintraege"."created_at" AS t1_r6, "journal_eintraege"."updated_at" AS t1_r7, "journal_eintraege"."frist_bis" AS t1_r8, "journal_eintraege"."kommission_refs" AS t1_r9, "geschaefte"."id" AS t2_r0, "geschaefte"."nummer" AS t2_r1, "geschaefte"."titel" AS t2_r2, "geschaefte"."eingereicht_am" AS t2_r3, "geschaefte"."zustaendig_refs" AS t2_r4, "geschaefte"."created_at" AS t2_r5, "geschaefte"."updated_at" AS t2_r6, "geschaefte"."grad" AS t2_r7, "geschaefte"."status" AS t2_r8, "geschaefte"."eingereicht_von_refs" AS t2_r9, "geschaefte"."zum_abschluss_vorgeschlagen" AS t2_r10, "geschaefte"."cache_frist_bis" AS t2_r11, "geschaefte"."type" AS t2_r12, "geschaefte"."art_ref" AS t2_r13, "geschaefte"."quartalsplanung_ref" AS t2_r14, "geschaefte"."cache_stand_ref" AS t2_r15, "geschaefte"."cache_aktueller_eintrag_id" AS t2_r16, "geschaefte"."cache_kommissionen_refs" AS t2_r17, "geschaefte"."cache_publikation" AS t2_r18 FROM "dokumente" INNER JOIN "journal_eintraege" ON "journal_eintraege"."id" = "dokumente"."journal_eintrag_id" INNER JOIN "geschaefte" ON "geschaefte"."id" = "journal_eintraege"."geschaeft_id" AND (geschaefte.status <> 'entfernt') WHERE "dokumente"."versanddatum_id" = $1 ORDER BY geschaefte.nummer, regexp_replace(dokumente.nummer, '^([[:digit:]]*).*$', '\1')::bigint DESC, regexp_replace(dokumente.nummer, '^.*?([^[:digit:]]*)$', '\1') DESC
```

https://errbit.4teamwork.ch/apps/52fb8ced3838181a800040dd/problems/53da43fd383818ee890003f7