get_process_environ: exit early when there is nothing to read

src/basic/process-util.c

@@ -329,6 +329,9 @@ int get_process_environ(pid_t pid, char **env) {
                         sz += cescape_char(c, outcome + sz);
         }
 
+        if (sz == 0)
+                return -ENOENT;
+
         outcome[sz] = '\0';
         *env = outcome;
         outcome = NULL;