fix: Send empty value sends empty string instead of omitting empty array params

[mixelburg]

Problem

When a multipart/form-data request has an array parameter (e.g., files: array<string>) with no items and the user checks the "Send empty value" checkbox, Swagger UI sends -F 'files=' (an empty string) instead of either omitting the field or sending an empty array.

This causes servers to respond with 422 Unprocessable Entity because an empty string is not a valid value for the expected array type.

Reported in #10785.

Root Cause

In src/core/plugins/spec/actions.js, the executeRequest filter logic was using requestBodyInclusionSetting.get(key) to override the empty check for all value types. When the "send empty value" flag was checked:

1. Empty arrays ([]) passed through the filter
2. swagger-client's formatKeyValue converted [] to an empty string via .join(",")
3. The request was sent with -F 'files=' (empty string)

Fix

Updated the filter logic to handle arrays and nil values correctly:

• Empty arrays: always excluded (never serialized as empty strings)
• Nil values (null/undefined): only included when the "send empty" flag is set AND the value is not nil (prevents sending empty strings for nil values)
• Empty strings: still properly handled by the inclusion flag

Testing

1. Define a multipart/form-data endpoint with an array<string> parameter
2. Open "Try it out" — do NOT add any items
3. Check "Send empty value"
4. Click Execute
5. Verify the curl output does not contain -F 'files='