strv: move nulstr utilities to nulstr-util.[ch]

Let's move them out of the generic, already very long strv.[ch] module
into the more specific nulst-util.[ch]

No code changes.

src/shared/ask-password-api.c

@@ -34,6 +34,7 @@
 #include "memory-util.h"
 #include "missing_syscall.h"
 #include "mkdir-label.h"
+#include "nulstr-util.h"
 #include "process-util.h"
 #include "random-util.h"
 #include "signal-util.h"

src/shared/condition.c

@@ -34,12 +34,13 @@
 #include "fs-util.h"
 #include "glob-util.h"
 #include "hostname-util.h"
-#include "initrd-util.h"
 #include "ima-util.h"
+#include "initrd-util.h"
 #include "limits-util.h"
 #include "list.h"
 #include "macro.h"
 #include "mountpoint-util.h"
+#include "nulstr-util.h"
 #include "os-util.h"
 #include "parse-util.h"
 #include "path-util.h"