fix(bevy_core_pipeline): remove early return in 2d main opaque pass

[MickHarrigan]

Objective

Fixes clear_color example that was broken due to #24850 by requiring 2d opaque passes to not early return, as referenced by this comment.

Solution

Removed the early return in main_opaque_pass_2d() such that it follows the same general shape as main_opaque_pass_3d().
Again, this comes from @beicause's comment:

just a heads up, this PR breaks the clear_color example. Just confirmed on both macos and linux personally

Both opaque_pass_2d and transparent_pass_2d skip render pass when there are no items. But at least one of them should always run to clear the texture.

Although the reason to always run transparent_pass_2d could be that transparent objects are more common than opaque objects in 2D, I think it's better to make opaque_pass_2d always run to be consistent with opaque_pass_3d.

Testing

Tested the aforementioned clear_color example which now works again as expected, along with the 2d_shapes and transparency_2d examples.

[github-actions]

Welcome, new contributor!

Please make sure you've read our contributing guide, as well as our policy regarding AI usage, and we look forward to reviewing your pull request shortly ✨

[kfc35]

Tested and verified, and I agree with the solution