Fresh synced node panics when HotFeed routine starts

```HYPERSYNC=true
RUN_HOT_FEED_ROUTINE=true
TXINDEX=true
ARCHIVAL_MODE=false

I0607 17:10:48.403583 1 hot_feed.go:102] Refreshing hot feed...
I0607 17:10:48.404506 1 hot_feed.go:405] UpdateHotFeedOrderedList: Starting new update cycle.
panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x8 pc=0xf2ce83]

goroutine 152235 [running]:
github.com/deso-protocol/backend/routes.(*APIServer).PopulateHotnessInfoMap(0xc009f86000, 0xc0cc537200?, 0xc000190fd0?, 0xc0be009c58?, 0x1?, {0xc0c1d80000, 0x4381, 0xc0be009ad0?})
/deso/src/backend/routes/hot_feed.go:538 +0x103
github.com/deso-protocol/backend/routes.(*APIServer).UpdateHotFeedOrderedList(0xc009f86000, 0x2?, 0xc00076c000?)
/deso/src/backend/routes/hot_feed.go:468 +0xdef
github.com/deso-protocol/backend/routes.(*APIServer).UpdateHotFeed(0xc009f86000, 0x0)
/deso/src/backend/routes/hot_feed.go:120 +0x431
github.com/deso-protocol/backend/routes.(*APIServer).StartHotFeedRoutine.func1()
/deso/src/backend/routes/hot_feed.go:91 +0x39
created by github.com/deso-protocol/backend/routes.(*APIServer).StartHotFeedRoutine
/deso/src/backend/routes/hot_feed.go:81 +0x1ad```

Reproduce the panic with the HYPERSYNC, RUN_HOT_FEED_ROUTINE, TXINDEX, and ARCHIVAL_MODE settings shown. Start at PopulateHotnessInfoMap in backend/routes/hot_feed.go:538, then trace its callers at lines 468, 120, and 91. Done means a fresh synced node can start the hot-feed routine without this nil-pointer panic.

Written by the indexing model from the issue text.