fix(rgbgradient): raise a clear error for an empty color gradient

[eeshsaxena]

Bug

_handle_rgbgradient_dict() (rivalcfg/handlers/rgbgradient.py) accesses gradient[-1] in its smooth-gradient step without checking that any color was parsed:

# Smooth gradient (if possible) by adding a final color
if len(gradient) < 14 and gradient[-1]["pos"] != 100:
    gradient.append({"pos": 100, "color": gradient[0]["color"]})

A dict with no "colors" key, or "colors": [], is reachable through the documented Python dict API, and it makes gradient empty. gradient[-1] then raises IndexError: list index out of range before process_value() can reach its own if len(gradient) == 0: raise ValueError("no color: ...") check just below.

rgbgradientv2 is affected too, since its process_value() reuses this same function.

from rivalcfg.handlers import rgbgradient
setting_info = {"rgbgradient_header": {...}, "led_id": 1}

rgbgradient.process_value(setting_info, {"duration": 1000})
# IndexError: list index out of range   (expected: ValueError "no color")

rgbgradient.process_value(setting_info, {"colors": []})
# IndexError: list index out of range

Fix

Guard the smooth-gradient step with if gradient and .... The empty case now falls through to the existing "no color" ValueError in both rgbgradient and rgbgradientv2, which is the intended behavior. Valid single colors, gradients, and tuples are unaffected.

Tests

Adds regression tests to test/handlers/test_rgbgradient.py and test/handlers/test_rgbgradientv2.py (dict without a colors key, and empty colors). They fail on master (IndexError) and pass with this change; the full gradient handler suite stays green (86 passed). black and flake8 are clean.

[eeshsaxena]

Hi! Gentle nudge on this one whenever you have some bandwidth. It's a small, self-contained fix (fix(rgbgradient): raise a clear error for an empty color gradient), and it's currently mergeable with no conflicts. No urgency at all, and I'm happy to make any changes you'd like. Thanks for maintaining rivalcfg!