extraToCSL(): Fix "zoteroField[1] is undefined" on single-character field

E.g., "a:" on a line

https://forums.zotero.org/discussion/82286/new-line-in-extra-field-induces-error-during-bibliography-generation

test/tests/citeTest.js

@@ -41,5 +41,10 @@ describe("Zotero.Cite", function () {
 			var str2 = 'DOI: 10.0/abc';
 			assert.equal(Zotero.Cite.extraToCSL(str1), str2);
 		});
+		
+		it("should handle a single-character field name", function () {
+			var str = 'a: ';
+			assert.equal(Zotero.Cite.extraToCSL(str), str);
+		});
 	});
 });