Made it illegal to call Begin("") with an empty string. This somehow accidentally worked before but had various undesirable side-effect as the window would have ID zero. In particular it is causing problems in viewport/docking branches.

docs/CHANGELOG.txt

@@ -37,6 +37,8 @@ Breaking Changes:
 
 - Renamed io.ConfigResizeWindowsFromEdges to io.ConfigWindowsResizeFromEdges and removed its [Beta] mark.
   The addition of new configuration options in the Docking branch is pushing for a little reorganization of those names.
+- Made it illegal to call Begin("") with an empty string. This somehow accidentally worked before but had various 
+  undesirable side-effect as the window would have ID zero. In particular it is causing problems in viewport/docking branches.
 
 Other Changes:
 - Added BETA api for Tab Bar/Tabs widgets: (#261, #351)

imgui.cpp

@@ -4621,7 +4621,7 @@ bool ImGui::Begin(const char* name, bool* p_open, ImGuiWindowFlags flags)
 {
     ImGuiContext& g = *GImGui;
     const ImGuiStyle& style = g.Style;
-    IM_ASSERT(name != NULL);                        // Window name required
+    IM_ASSERT(name != NULL && name[0] != '\0');     // Window name required
     IM_ASSERT(g.FrameScopeActive);                  // Forgot to call ImGui::NewFrame()
     IM_ASSERT(g.FrameCountEnded != g.FrameCount);   // Called ImGui::Render() or ImGui::EndFrame() and haven't called ImGui::NewFrame() again yet