fix: Use nullish coalescing for Content-Type header

Replaced logical OR (||) with nullish coalescing operator (??)
for fetching the 'Content-Type' header in `DefaultResponseProcess`.

This change makes the assignment safer, as `??` only falls back
to the default value (empty string) if `response.headers.get('Content-Type')`
is strictly `null` or `undefined`, aligning with modern JavaScript best practices.

Author: KariHall619

console/atest-ui/src/views/net.ts

@@ -42,7 +42,7 @@ async function DefaultResponseProcess(response: any): Promise<any> {
     }
 
     // Check if the content type is explicitly text/plain
-    const contentType = response.headers.get('Content-Type') || '';
+    const contentType = response.headers.get('Content-Type') ?? '';
     if (contentType.startsWith('text/plain')) {
         // For text/plain, directly return the text without parsing
         return await response.text();