fix: allow screenshot capture when filename is an empty string

[ogadra]

Problem

• Screenshot saving failed when an empty string ("") was passed as the filename.
• The resolver treated the empty string as a path to a directory end and incorrectly flagged it as file path for is outside returning an error response instead of an image.

Root Cause

• The logic for assigning a default filename handled cases where the parameter was omitted, but it did not account for cases where an empty string was explicitly provided.

Fix

• Treat empty filename as null.
• Let the existing defaulting path assign the default filename (e.g., page-YYYY-MM-DDTHH-mm-ss-SSSZ.png) in the output directory.

Behavior Changes

• Before: empty string → error response.
• After: empty string → screenshot is saved with a default filename, normal success payload.

[yury-s]

When is it called with an empty string instead of omitting the parameter?

[yury-s]

just change ?? to || at line 54:

const fileName = await tab.context.outputFile(params.filename || dateAsFileName(fileType), { origin: 'llm', reason: 'Saving screenshot' }); 

[ogadra]

@yury-s Thanks for reviewing!

I'm using this MCP server with the GPT-5 model via Mastra. When the filename is omitted in the prompt, the model sometimes calls the function with filename: "", which appears to be a model hallucination. While switching models is a possible workaround, I suggest that this edge case be handled within the library for improved robustness.