openPMD Engines: Allow Params w/o Type (ECP-WarpX#3460)

The current implementation only accepted parameters for ADIOS2
engines if the engine type was also specified. That is not
necessary, the default (bp4) will usually be taken and thus
the documentation did not work.

This fixes it, keeping the engine type optional in the inputs.

Source/Diagnostics/WarpXOpenPMD.cpp

@@ -181,34 +181,44 @@ namespace detail
                 op_block += R"END(,
           "parameters": {
 )END";
-            op_block += op_parameters + "}";
+            op_block += op_parameters +
+                        "\n          }";
         }
             op_block += R"END(
         }
       ]
     })END";
-        if (!engine_type.empty())
-            op_block += ",";
-
+            if (!engine_type.empty() || !en_parameters.empty())
+                op_block += ",";
         }  // end operator string block
 
         // add the engine string block
-        if (!engine_type.empty()) {
+        if (!engine_type.empty() || !en_parameters.empty())
+        {
             en_block = R"END(
-    "engine": {
+    "engine": {)END";
+
+            // non-default engine type
+            if (!engine_type.empty()) {
+                en_block += R"END(
       "type": ")END";
-            en_block += engine_type + "\"";
+                en_block += engine_type + "\"";
 
+                if(!en_parameters.empty())
+                    en_block += ",";
+            }
+
+            // non-default engine parameters
             if (!en_parameters.empty()) {
-                en_block += R"END(,
+                en_block += R"END(
       "parameters": {
 )END";
-            en_block += en_parameters + "}";
+                en_block += en_parameters +
+                            "\n      }";
             }
 
             en_block += R"END(
     })END";
-
         }  // end engine string block
 
         options = top_block + op_block + en_block + end_block;