fixed an exception in case of no text in the returned synthaxBase value

Author: ouldsid

src/Bicep.Core/Emit/PlaceholderParametersJsonWriter.cs

@@ -101,7 +101,11 @@ private JToken GenerateTemplate(string contentVersion)
 
                     jsonWriter.WriteStartObject();
 
-                    emitter.EmitProperty("value", value);
+                    // emit value property only if the text is not null
+                    if (value.ToString() != "\r\n")
+                    {
+                        emitter.EmitProperty("value", value);
+                    }
 
                     jsonWriter.WriteEndObject();
                 }