Fix room pickers not loading more than the first page of rooms

The forward, share, add-to-space and search pickers signal reaching the
bottom of the list with an empty `roomCount..<roomCount` range. The
prefetch sink added in 9dd9fd917 guards on `!range.isEmpty` to skip the
initial `0..<0`, which also dropped the pickers' bottom signal, so they
never loaded past the first 100 rooms.

Only skip `0..<0` instead, which leaves the home screen prefetch as it
was, and cover the bottom, prefetch and top cases in
RoomSummaryProviderTests.

ElementX/Sources/Services/Room/RoomSummary/RoomSummaryProvider.swift

@@ -192,7 +192,8 @@ class RoomSummaryProvider: RoomSummaryProviderProtocol {
         visibleItemRangePublisher
             .sink { [weak self] range in
                 guard let self,
-                      !range.isEmpty,
+                      // Pickers send an empty `count..<count` range when reaching the bottom, so only ignore `0..<0` (the initial value or the top).
+                      range.upperBound > 0,
                       range.upperBound >= rooms.count - Int(roomListPageSize) / 2,
                       rooms.count != roomCountOnLastPageAddRequest else {
                     return