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/build/derivation-goal.cc

@@ -1505,7 +1505,7 @@ std::pair<bool, SingleDrvOutputs> DerivationGoal::checkPathValidity()
     if (!wantedOutputsLeft.empty())
         throw Error("derivation '%s' does not have wanted outputs %s",
             worker.store.printStorePath(drvPath),
-            dropEmptyInitThenConcatStringsSep(", ", quoteStrings(wantedOutputsLeft)));
+            concatStringsSep(", ", quoteStrings(wantedOutputsLeft)));
 
     bool allValid = true;
     for (auto & [_, status] : initialOutputs) {

src/libstore/outputs-spec.cc

@@ -5,6 +5,7 @@
 #include "regex-combinators.hh"
 #include "outputs-spec.hh"
 #include "path-regex.hh"
+#include "strings-inline.hh"
 
 namespace nix {
 
@@ -83,7 +84,7 @@ std::string OutputsSpec::to_string() const
             return "*";
         },
         [&](const OutputsSpec::Names & outputNames) -> std::string {
-            return dropEmptyInitThenConcatStringsSep(",", outputNames);
+            return concatStringsSep(",", outputNames);
         },
     }, raw);
 }

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);
 }