fix(rules): Don't trip on ebook metadata when source identified but n…

…ot with a title

rules/ebooks.mk

@@ -12,7 +12,7 @@ DISTFILES += $(EPUBS)
 
 $(BUILDDIR)/%-epub-metadata.yml: $$(call parse_bookid,$$*)-manifest.yml %-epub-$(_poster).jpg | $(BUILDDIR)
 	echo '---' > $@
-	$(YQ) -M -e -y '{title: [ { type: "main", text: .title  }, { type: "subtitle", text: .subtitle } ], creator: .creator, contributor: .contributor, identifier: .identifier, date: .date | last | .text, published: .date | first | .text, lang: .lang, description: .abstract, rights: .rights, publisher: .publisher, source: (if .source then (.source[]? | select(.type == "title").text) else null end), "cover-image": "$(filter %.jpg,$^)" }' < $< >> $@
+	$(YQ) -M -e -y '{title: [ { type: "main", text: .title  }, { type: "subtitle", text: .subtitle } ], creator: .creator, contributor: .contributor, identifier: .identifier, date: .date | last | .text, published: .date | first | .text, lang: .lang, description: .abstract, rights: .rights, publisher: .publisher, source: (if .source then (try (.source[]? | map(select(.type == "title"))[0].text) // null) else null end), "cover-image": "$(filter %.jpg,$^)" }' < $< >> $@
 	echo '...' >> $@
 
 MOBIS := $(call pattern_list,$(EDITIONEDITSOURCES),.mobi)