Fix: Axes submobject colors are not being set properly

[ishu9bansal]

Overview: What does this pull request change?

Mitigates the issue, that setting the color of a DecimalNumber object to BLACK will actually render a number with WHITE fill.
Further details about the issue can be found on the issue itself #3633

Motivation and Explanation: Why and how do your changes improve the library?

Root Cause:
The method get_fill_colors treats the RGBA value [0, 0, 0, 0] (fully transparent black) as equivalent to None.
As a result, during the initialization of certain objects, the fill_color is effectively set to None, while in other cases it defaults to WHITE.
Therefore, when we explicitly set fill_color to BLACK, it ends up being interpreted as WHITE due to this inconsistency.

The code that interprets [0,0,0,0] as None can be found here:

manim/manim/mobject/types/vectorized_mobject.py

Lines 566 to 573 in 3377f6c

	# TODO: Does this just do a copy?
	# TODO: I have the feeling that this function should not return None, does that have any usage ?
	def get_fill_colors(self) -> list[ManimColor | None]:
	return [
	ManimColor(rgba[:3]) if rgba.any() else None
	for rgba in self.get_fill_rgbas()
	]

Mitigation:
Initialize the VMobject with a fill_opacity value from the input parameters (which defaults to 1 for a DecimalNumber object). This ensures the fill_color is applied correctly.
By default, fill_opacity is 0 for a VMobject, which causes the internal fill_color to be [0, 0, 0, 0] (interpreted as None) even when BLACK is specified.
Setting fill_opacity = 1 ensures that the fill_color is preserved as BLACK as intended.

Note:
This is a workaround. The underlying issue is that a fill_color of BLACK is effectively ignored when fill_opacity = 0, due to its [0, 0, 0, 0] representation.
We should investigate whether this behavior is expected or a design flaw, and address it in a separate issue.

Reviewer Checklist

• The PR title is descriptive enough for the changelog, and the PR is labeled correctly
• If applicable: newly added non-private functions and classes have a docstring including a short summary and a PARAMETERS section
• If applicable: newly added functions and classes are tested

[behackl]

This workaround seems fine to me, thank you.