Fix infinite loop when ANSI escape sequences appear at string start

[veeceey]

Summary

Fixes #3958

When strings with ANSI escape sequences at the start were printed using console.print(), the program would hang in an infinite loop. This was a critical bug introduced in version 14.3.2.

Root Cause

The issue was in split_graphemes() in rich/cells.py:

1. ANSI escape characters (\x1b) have zero cell width
2. Zero-width characters are normally associated with the previous character
3. When no previous character exists (at string start), the index wasn't incremented
4. This caused an infinite loop in the while loop at line 182

Changes

• Added an else clause to handle zero-width characters when spans is empty
• When there's no previous character to associate with, skip the zero-width character
• Added test test_ansi_at_start() to verify the fix

Test plan

• Added regression test that verifies no infinite loop occurs
• All existing cell tests pass
• Manual testing with the exact strings from the issue report confirms they now print without hanging

Before

from rich.console import Console

Console().print('\x1b[38;5;249mi\x1b[0m')  # Hangs forever

After

from rich.console import Console

Console().print('\x1b[38;5;249mi\x1b[0m')  # Prints successfully

[TomJGooding]

I notice you also opened #3959 for the same issue. Was this PR created by mistake?