dropEmptyInitThenConcatStringsSep -> concatStringSep: PATH handling

It's still wrong, but one step closer to correct.
Not that anyone should use "" or "." in their PATH, but that is not
for us to intervene.

src/nix/env.cc

@@ -3,6 +3,7 @@
 
 #include "command.hh"
 #include "run.hh"
+#include "strings.hh"
 
 using namespace nix;
 
@@ -92,9 +93,10 @@ struct CmdShell : InstallablesCommand, MixEnvironment
             }
         }
 
+        // TODO: split losslessly; empty means .
         auto unixPath = tokenizeString<Strings>(getEnv("PATH").value_or(""), ":");
         unixPath.insert(unixPath.begin(), pathAdditions.begin(), pathAdditions.end());
-        auto unixPathString = dropEmptyInitThenConcatStringsSep(":", unixPath);
+        auto unixPathString = concatStringsSep(":", unixPath);
         setEnv("PATH", unixPathString.c_str());
 
         Strings args;