Fix uncaught exceptions (#414)

* 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)
```

src/main.ts

@@ -1,4 +1,11 @@
-import { App, Notice, Platform, Plugin, type PluginManifest } from 'obsidian'
+import {
+  App,
+  Notice,
+  Platform,
+  Plugin,
+  type PluginManifest,
+  TFile,
+} from 'obsidian'
 import {
   OmnisearchInFileModal,
   OmnisearchVaultModal,
@@ -109,7 +116,10 @@ export default class OmnisearchPlugin extends Plugin {
       // Listeners to keep the search index up-to-date
       this.registerEvent(
         this.app.vault.on('create', file => {
-          if (this.notesIndexer.isFileIndexable(file.path)) {
+          if (
+            file instanceof TFile &&
+            this.notesIndexer.isFileIndexable(file.path)
+          ) {
             logDebug('Indexing new file', file.path)
             searchEngine.addFromPaths([file.path])
             this.embedsRepository.refreshEmbedsForNote(file.path)

src/repositories/documents-repository.ts

@@ -97,17 +97,18 @@ export class DocumentsRepository {
     // ** Canvas **
     // Extract the text fields from the json
     else if (isFileCanvas(path)) {
-      const canvas = JSON.parse(await app.vault.cachedRead(file)) as CanvasData
+      const fileContents = await app.vault.cachedRead(file)
+      const canvas: CanvasData = fileContents ? JSON.parse(fileContents) : {}
       let texts: string[] = []
       // Concatenate text from the canvas fields
-      for (const node of canvas.nodes) {
+      for (const node of canvas.nodes ?? []) {
         if (node.type === 'text') {
           texts.push(node.text)
         } else if (node.type === 'file') {
           texts.push(node.file)
         }
       }
-      for (const edge of canvas.edges.filter(e => !!e.label)) {
+      for (const edge of (canvas.edges ?? []).filter(e => !!e.label)) {
         texts.push(edge.label!)
       }
       content = texts.join('\r\n')