Ninja: Omit custom commands with an empty COMMAND

Fixes: #21063

Source/cmLocalNinjaGenerator.cxx

@@ -557,9 +557,13 @@ void cmLocalNinjaGenerator::AppendCustomCommandLines(
   std::string launcher = this->MakeCustomLauncher(ccg);
 
   for (unsigned i = 0; i != ccg.GetNumberOfCommands(); ++i) {
+    std::string c = ccg.GetCommand(i);
+    if (c.empty()) {
+      continue;
+    }
     cmdLines.push_back(launcher +
                        this->ConvertToOutputFormat(
-                         ccg.GetCommand(i),
+                         c,
                          gg->IsMultiConfig() ? cmOutputConverter::NINJAMULTI
                                              : cmOutputConverter::SHELL));
 

Tests/CustomCommand/CMakeLists.txt

@@ -582,3 +582,7 @@ set_target_properties(mac_fw PROPERTIES
   )
 add_custom_command(OUTPUT mac_fw.txt COMMAND ${CMAKE_COMMAND} -E touch mac_fw.txt DEPENDS mac_fw)
 add_custom_target(drive_mac_fw ALL DEPENDS mac_fw.txt)
+
+# Test empty COMMANDs are ommited
+add_executable(empty_command empty_command.cxx)
+add_custom_command(TARGET empty_command POST_BUILD COMMAND $<0:date>)

Tests/CustomCommand/empty_command.cxx

@@ -0,0 +1,4 @@
+int main()
+{
+  return 0;
+}