Refactor to enforce strictNullChecks

This commit applies `strictNullChecks` to the entire codebase to improve
maintainability and type safety. Key changes include:

- Remove some explicit null-checks where unnecessary.
- Add necessary null-checks.
- Refactor static factory functions for a more functional approach.
- Improve some test names and contexts for better debugging.
- Add unit tests for any additional logic introduced.
- Refactor `createPositionFromRegexFullMatch` to its own function as the
  logic is reused.
- Prefer `find` prefix on functions that may return `undefined` and
  `get` prefix for those that always return a value.

src/presentation/electron/main/Update/ManualUpdater.ts

@@ -104,10 +104,14 @@ async function downloadFileWithProgress(
   if (!response.ok) {
     throw Error(`Unable to download, server returned ${response.status} ${response.statusText}`);
   }
-  const contentLength = +response.headers.get('content-length');
+  const contentLengthString = response.headers.get('content-length');
+  if (contentLengthString === null || contentLengthString === undefined) {
+    log.error('Content-Length header is missing');
+  }
+  const contentLength = +(contentLengthString ?? 0);
   const writer = fs.createWriteStream(filePath);
   log.info(`Writing to ${filePath}, content length: ${contentLength}`);
-  if (!contentLength) {
+  if (Number.isNaN(contentLength) || contentLength <= 0) {
     log.error('Unknown content-length', Array.from(response.headers.entries()));
     progressHandler = () => { /* do nothing */ };
   }
@@ -138,7 +142,7 @@ async function streamWithProgress(
   log.info('Downloaded successfully');
 }
 
-function getReader(response: Response): NodeJS.ReadableStream {
+function getReader(response: Response): NodeJS.ReadableStream | undefined {
   // On browser, we could use browser API response.body.getReader()
   // But here, we use cross-fetch that gets node-fetch on a node application
   // This API is node-fetch specific, see https://github.com/node-fetch/node-fetch#streams

src/presentation/electron/main/Update/UpdateProgressBar.ts

@@ -4,7 +4,7 @@ import { app, BrowserWindow } from 'electron';
 import log from 'electron-log';
 
 export class UpdateProgressBar {
-  private progressBar: ProgressBar;
+  private progressBar: ProgressBar | undefined;
 
   private get innerProgressBarWindow(): BrowserWindow {
     // eslint-disable-next-line no-underscore-dangle
@@ -39,16 +39,16 @@ export class UpdateProgressBar {
         + `\n${e && e.message ? e.message : e}`;
       this.innerProgressBarWindow.setClosable(true);
     };
-    if (this.progressBar.innerProgressBarWindow) {
+    if (this.progressBar?.innerProgressBarWindow) {
       reportUpdateError();
     } else {
-      this.progressBar.on('ready', () => reportUpdateError());
+      this.progressBar?.on('ready', () => reportUpdateError());
     }
   }
 
   public close() {
-    if (!this.progressBar.isCompleted()) {
-      this.progressBar.close();
+    if (!this.progressBar?.isCompleted()) {
+      this.progressBar?.close();
     }
   }
 }