Preserve falsy parsed outputs

[sylvesterkaczmarek]

Summary

Preserve valid falsy structured outputs returned by messages.parse() and beta.messages.parse().

Both ParsedMessage.parsed_output and ParsedBetaMessage.parsed_output currently use a truthiness check:

if content.type == "text" and content.parsed_output:

As a result, valid parsed outputs such as False, 0, "", [], and {} are incorrectly exposed as None.

Fix

Check explicitly for None instead of truthiness:

if content.type == "text" and content.parsed_output is not None:

This preserves all valid structured-output values while continuing to return None when no parsed output is present.

The fix is applied consistently to both:

• ParsedMessage
• ParsedBetaMessage

Regression coverage

Adds focused tests covering:

• False
• 0
• empty string
• empty list
• empty dict
• genuinely absent parsed_output

The regression also verifies that an earlier text block with parsed_output=None does not prevent a later valid falsy parsed value from being returned.