Fix empty name error

It is allowed to mount series in Tobira without providing a name
in the UI. That makes sense since that is always overridden by
the series name (after storing it in the DB).

But leaving out the name would lead to a DB error before its overridden,
since an empty string was passed and rust doesn't infer that as `null`
(in which case it would be overridden like it should).
Passing `null` instead of an empty string fixes this.

src/slices/seriesSlice.ts

@@ -224,7 +224,7 @@ export const postNewSeries = createAppAsyncThunk('series/postNewSeries', async (
 		values.breadcrumbs.concat(values.selectedPage).forEach( function (page: TobiraPage) {
 			if (page.new) {
 				newPages.push({
-					name: page.title,
+					name: page.title !== "" ? page.title : null,
 					pathSegment: page.segment,
 				});
 			} else {