shared/json: use different return code for empty input

It is useful to distinguish if json_parse_file() got no input or invalid input.
Use different return codes for the two cases.

src/shared/elf-util.c

@@ -859,7 +859,7 @@ int parse_elf_object(int fd, const char *executable, bool fork_disable_dump, cha
                         return -errno;
 
                 r = json_parse_file(json_in, NULL, 0, &package_metadata, NULL, NULL);
-                if (r < 0 && r != -EINVAL) /* EINVAL: json was empty, so we got nothing, but that's ok */
+                if (r < 0 && r != -ENODATA) /* ENODATA: json was empty, so we got nothing, but that's ok */
                         return r;
         }
 

src/shared/json.c

@@ -3187,7 +3187,6 @@ int json_parse_continue(const char **p, JsonParseFlags flags, JsonVariant **ret,
 int json_parse_file_at(FILE *f, int dir_fd, const char *path, JsonParseFlags flags, JsonVariant **ret, unsigned *ret_line, unsigned *ret_column) {
         _cleanup_(json_source_unrefp) JsonSource *source = NULL;
         _cleanup_free_ char *text = NULL;
-        const char *p;
         int r;
 
         if (f)
@@ -3199,13 +3198,16 @@ int json_parse_file_at(FILE *f, int dir_fd, const char *path, JsonParseFlags fla
         if (r < 0)
                 return r;
 
+        if (isempty(text))
+                return -ENODATA;
+
         if (path) {
                 source = json_source_new(path);
                 if (!source)
                         return -ENOMEM;
         }
 
-        p = text;
+        const char *p = text;
         return json_parse_internal(&p, source, flags, ret, ret_line, ret_column, false);
 }