Fix Live leaving blank line for empty renderables

[Faheem12005]

Type of changes

• Bug fix
• New feature
• Documentation / docstrings
• Tests
• Other

AI?

• AI was used to generate this PR

AI generated PRs may be accepted, but only if @willmcgugan has responded on an issue or discussion.

Checklist

• I've run the latest black with default args on new code.
• I've updated CHANGELOG.md and CONTRIBUTORS.md where appropriate (see note about typos above).
• I've added tests for new code.
• I accept that @willmcgugan may be pedantic in the code review.

Description

When Live is used with an empty renderable like an empty list, it still emits a newline because an empty renderable is treated as producing one renderable line. As a result, Live advances the cursor and leaves behind a blank line even though nothing was rendered.

This PR fixes the issue by detecting empty renderables during the render phase and preventing Live from advancing the cursor when no renderable output is produced.

Can be tested with this snippet:

from rich.live import Live
from rich.console import Console

console = Console()

with Live([], console=console):
    pass

Fixes #3796