Try to fix build failure caused by r322097

Avoid mapping during output when Detail is nullptr; otherwise, an empty
"Detail" field will be populated in YAML output.

llvm-svn: 322116

clang-tools-extra/clangd/index/SymbolYAML.cpp

@@ -92,6 +92,7 @@ template <> struct MappingTraits<Symbol> {
 
     IO.mapOptional("CompletionSnippetInsertText",
                    Sym.CompletionSnippetInsertText);
+    if (!IO.outputting() || Sym.Detail)
       IO.mapOptional("Detail", Sym.Detail);
   }
 };