dropEmptyInitThenConcatStringsSep -> concatStringSep: diagnostics and docs

These are non-critical, so their behavior is ok to change.
Dropping empty items is not needed and usually not expected.

src/libcmd/command.cc

@@ -1,3 +1,5 @@
+#include <nlohmann/json.hpp>
+
 #include "command.hh"
 #include "markdown.hh"
 #include "store-api.hh"
@@ -6,8 +8,7 @@
 #include "nixexpr.hh"
 #include "profiles.hh"
 #include "repl.hh"
-
-#include <nlohmann/json.hpp>
+#include "strings.hh"
 
 extern char * * environ __attribute__((weak));
 
@@ -42,7 +43,7 @@ void NixMultiCommand::run()
         for (auto & [name, _] : commands)
             subCommandTextLines.insert(fmt("- `%s`", name));
         std::string markdownError = fmt("`nix %s` requires a sub-command. Available sub-commands:\n\n%s\n",
-                commandName, dropEmptyInitThenConcatStringsSep("\n", subCommandTextLines));
+                commandName, concatStringsSep("\n", subCommandTextLines));
         throw UsageError(renderMarkdownToTerminal(markdownError));
     }
     command->second->run();

src/libcmd/installables.cc

@@ -27,6 +27,8 @@
 
 #include <nlohmann/json.hpp>
 
+#include "strings-inline.hh"
+
 namespace nix {
 
 void completeFlakeInputPath(
@@ -630,7 +632,7 @@ static void throwBuildErrors(
                 }
                 failedPaths.insert(failedResult->path.to_string(store));
             }
-            throw Error("build of %s failed", dropEmptyInitThenConcatStringsSep(", ", quoteStrings(failedPaths)));
+            throw Error("build of %s failed", concatStringsSep(", ", quoteStrings(failedPaths)));
         }
     }
 }

src/libcmd/repl.cc

@@ -33,6 +33,8 @@
 #include <gc/gc_cpp.h>
 #endif
 
+#include "strings.hh"
+
 namespace nix {
 
 /**
@@ -625,7 +627,7 @@ ProcessLineResult NixRepl::processLine(std::string line)
 
                 markdown +=
                     "**Synopsis:** `builtins." + (std::string) (*doc->name) + "` "
-                    + dropEmptyInitThenConcatStringsSep(" ", args) + "\n\n";
+                    + concatStringsSep(" ", args) + "\n\n";
             }
 
             markdown += stripIndentation(doc->doc);