dropEmptyInitThenConcatStringsSep -> concatStringSep: output name empty not feasible

I don't think it's completely impossible, but I can't construct
one easily as derivationStrict seems to (re)tokenize the outputs
attribute, dropping the empty output.

It's not a scenario we have to account for here.

src/libstore/path-with-outputs.cc

@@ -1,7 +1,9 @@
+#include <regex>
+
 #include "path-with-outputs.hh"
 #include "store-api.hh"
+#include "strings.hh"
 
-#include <regex>
 
 namespace nix {
 
@@ -9,7 +11,7 @@ std::string StorePathWithOutputs::to_string(const StoreDirConfig & store) const
 {
     return outputs.empty()
         ? store.printStorePath(path)
-        : store.printStorePath(path) + "!" + dropEmptyInitThenConcatStringsSep(",", outputs);
+        : store.printStorePath(path) + "!" + concatStringsSep(",", outputs);
 }