fix(agentflow): handle empty form object in form-input mode

[octo-patch]

Fixes #6013

Problem

When an AgentFlow V2 uses Form Input on the Start node, subsequent messages from the embed chat (or share link) arrive at the server with an empty form: {} payload and no question field:

{"chatId":"...","form":{},"overrideConfig":{},"streaming":true}

Because an empty object is truthy in JavaScript, the existing code branches treated form: {} as a populated form. This caused two issues:

1. finalInput was set to "" (empty string from Object.entries({}).join('\n')) instead of falling through to the user's question
2. finalUserInput (the content saved to the ChatMessage table) was also set to "", causing blank user messages to be stored in the database
3. The validation guard if (incomingInput.question && incomingInput.form) would erroneously throw if a caller sent question alongside an empty form: {}

Solution

Introduce an explicit incomingFormHasData check (Object.keys(form).length > 0) instead of relying on bare object truthiness:

• Only treat incomingInput.form as real form data when it actually contains keys
• Apply the same guard when computing finalUserInput for formInput flows
• Relax the mutual-exclusion validation so it only fires when both question and a non-empty form are present simultaneously

This allows the server to correctly fall back to the question field when an empty form: {} is sent, and stores a meaningful user message in the database.

Testing

• Verified the logic change with the three affected code paths in buildAgentflow.ts
• Existing form-input behaviour (non-empty form) is preserved: the new guard evaluates identically to the old truthy check for populated form objects

[gemini-code-assist]

Code Review

The pull request refactors form input handling in buildAgentflow.ts by introducing explicit checks for non-empty form data before processing. This includes creating incomingFormHasData in executeNode and adding Object.keys(form).length > 0 in executeAgentFlow, along with removing a redundant nullish coalescing in the latter. A review comment suggests that the || {} in executeNode is now also redundant and should be removed for consistency.

[gemini-code-assist]

Since incomingFormHasData already ensures that incomingInput.form is a non-null object, the nullish coalescing with {} is no longer necessary here. Removing it would also make this code consistent with the change in executeAgentFlow where a similar || {} was removed.

            finalInput = Object.entries(incomingInput.form)