Avoid creating a pass for main_transparent_pass_2d when unecessary

[PJB3005]

Objective

• Less CPU & GPU usage from an empty render pass

Solution

• Move the if condition to earlier

Testing

• Ran 2d_shapes and transparency_2d examples and it looks fine.

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Showcase

[mardzie]

The code looks good to me but I am not familiar with the broader context.

[beicause]

Makes sense.

We already skipped empty render pass for 3d

bevy/crates/bevy_core_pipeline/src/core_3d/main_transparent_pass_3d_node.rs

Line 50 in 306aaed

if !transparent_phase.items.is_empty() {

transparent 3d doesn't skip the webgl2 workaround though, but this is fine (#24851).

[PJB3005]

transparent 3d doesn't skip the webgl2 workaround though

Yeah I figured that'd be fine since we're not doing anything here.