fix: return 0 height inside GetLastBlockHeight if there are no blocks (…

…#76)

When we use the parse blocks command, in case that the database is empty, `return err` here will error out the program and not continuing parsing the desired block height. Replacing it here so it's more handy for dev purpose when we need to just test specific heights. (we don't have to run ` start cmd` before using `parse blocks all`)

- [x] Targeted PR against correct branch.
- [ ] Linked to Github issue with discussion and accepted design OR link to spec that describes this work.
- [ ] Wrote unit tests.
- [x] Re-reviewed `Files changed` in the Github PR explorer.

CHANGELOG.md

@@ -2,6 +2,7 @@
 ### Changes
 - ([\#74](https://github.com/forbole/juno/pull/74)) Added database block count to prometheus to improve alert monitoring
 - ([\#75](https://github.com/forbole/juno/pull/75)) Allow modules to handle MsgExec inner messages
+- ([\#76](https://github.com/forbole/juno/pull/76)) Return 0 as height for `GetLastBlockHeight()` method while no block is saved
 - Updated cosmos/cosmos-sdk to `v0.45.8`
 - Use `sqlx` instead of `sql` while dealing with a PostgreSQL database