Fix infinite loop with zero top solid infill density (#12762)

# Description

This pull request fixes the bug where Orca Slicer enters an infinite
loop when a density of zero is introduced for the top layer solid infill

# Screenshots/Recordings/Graphs

<img width="2560" height="1392" alt="image"
src="https://github.com/user-attachments/assets/5bf5c220-a398-483f-9f69-63dadbc888af"
/>

Note: For bottom solid infill, this is not a problem because the minimum
density is 10%.

src/libslic3r/Fill/Fill.cpp

@@ -882,6 +882,7 @@ std::vector<SurfaceFill> group_fills(const Layer &layer, LockRegionParam &lock_p
                         if (surface.is_top()) {
                             params.pattern = region_config.top_surface_pattern.value;
                             params.density = float(region_config.top_surface_density);
+                            if (params.density <= 0.0f) continue;
                         } else { // Surface is bottom
                             params.pattern = region_config.bottom_surface_pattern.value;
                             params.density = float(region_config.bottom_surface_density);