* Fix handling of folders on create trigger
When creating a directory in the vault, omnisearch attempts to index the directory as a document
`on('create', (file) => any, ...)` is triggered for both files and directories, so any triggers on `create` should handle the case where `file` is a `TFolder`.[^1]
[^1]: https://docs.obsidian.md/Reference/TypeScript+API/Vault/on('create')
For example, after running `mkdir foobar` in the vault directory, the
following exception is thrown:
```stacktrace
documents-repository.ts:53 Omnisearch: Error while adding "foobar" to live cache Error: Not a TFile: "foobar"
at DocumentsRepository.getAndMapIndexedDocument (documents-repository.ts:85:41)
at DocumentsRepository.addDocument (documents-repository.ts:43:30)
at DocumentsRepository.getDocument (documents-repository.ts:69:16)
at eval (search-engine.ts:82:63)
at Array.map (<anonymous>)
at SearchEngine.addFromPaths (search-engine.ts:81:15)
at eval (main.ts:114:26)
at e.tryTrigger (app.js:1:723011)
at e.trigger (app.js:1:722944)
at t.trigger (app.js:1:741049)
```
* Fix handling of empty canvas files
Canvas files can be empty. e.g. when the "Create new canvas" command is called:
- immediately after the command:
- a new *empty* .canvas file will be created
- triggering the `create` event and causing a "Unexpected end of JSON input" error
- when saving the file (<cmd/ctrl>+s or "Save current file" command):
- `{}` will be written to the file
- triggering a file `modify` event and causing a "canvas.nodes is not iterable" exception
Examples of the exceptions:
```stacktrace
documents-repository.ts:53 Omnisearch: Error while adding "Untitled.canvas" to live cache SyntaxError: Unexpected end of JSON input
at JSON.parse (<anonymous>)
at DocumentsRepository.getAndMapIndexedDocument (documents-repository.ts:100:27)
at async DocumentsRepository.addDocument (documents-repository.ts:43:19)
at async DocumentsRepository.getDocument (documents-repository.ts:69:5)
at async eval (search-engine.ts:82:25)
at async Promise.all (index 0)
at async SearchEngine.addFromPaths (search-engine.ts:80:7)
```
```stacktrace
documents-repository.ts:53 Omnisearch: Error while adding "Untitled.canvas" to live cache TypeError: canvas.nodes is not iterable
at DocumentsRepository.getAndMapIndexedDocument (documents-repository.ts:103:33)
at async DocumentsRepository.addDocument (documents-repository.ts:43:19)
at async NotesIndexer.refreshIndex (notes-indexer.ts:29:7)
```
