Visualizer: fix empty rows being shown (#2881)

Fixes a bug with the visualizer where there would be empty rows shown,
e.g. threads without any profiling regions. This was especially
noticable when going from a high thread count sample (MultiThread) to a
low thread count sample, where most of the visualizer would be empty
lines. This adds a data culling step to remove the threads without any
remaining execution data + an early out so that all threads shown
onscreen must have some regions recorded.

Signed-off-by: Jacob Hilliard <jhlliar@amazon.com>

Gems/Atom/Utils/Code/Include/Atom/Utils/ImGuiCpuProfiler.inl

@@ -329,7 +329,7 @@ namespace AZ
 
                 ImGui::Text("Viewport width: %.3f ms", CpuProfilerImGuiHelper::TicksToMs(GetViewportTickWidth()));
                 ImGui::Text("Ticks [%lld , %lld]", m_viewportStartTick, m_viewportEndTick);
-                ImGui::Text("Recording %ld threads", RHI::CpuProfiler::Get()->GetTimeRegionMap().size());
+                ImGui::Text("Recording %zu threads", m_savedData.size());
                 ImGui::Text("%llu profiling events saved", m_savedRegionCount);
 
                 ImGui::NextColumn();
@@ -389,6 +389,11 @@ namespace AZ
                             return wrapper.m_startTick < target;
                         });
 
+                    if (regionItr == singleThreadData.end())
+                    {
+                        continue;
+                    }
+
                     // Draw all of the blocks for a given thread/row
                     u64 maxDepth = 0;
                     while (regionItr != singleThreadData.end())
@@ -559,6 +564,14 @@ namespace AZ
 
                 m_savedRegionCount -= sizeBeforeRemove - savedRegions.size();
             }
+
+            // Remove any threads from the top-level map that no longer hold data
+            AZStd::erase_if(
+                m_savedData,
+                [](const auto& singleThreadDataEntry)
+                {
+                    return singleThreadDataEntry.second.empty();
+                });
         }
 
         inline void ImGuiCpuProfiler::DrawBlock(const TimeRegion& block, u64 targetRow)