sd-hwdb: allow empty properties

So far we didn't allow empty properties, but it makes sense to do so, for
example to distinguish empty data from lack of data. It also makes it easy to
override properties (back to the empty) value for specific cases.

src/libsystemd/sd-hwdb/hwdb-util.c

@@ -456,10 +456,9 @@ static int insert_data(struct trie *trie, char **match_list, char *line, const c
         while (isblank(line[0]) && isblank(line[1]))
                 line++;
 
-        if (isempty(line + 1) || isempty(value))
+        if (isempty(line + 1))
                 return log_syntax(NULL, LOG_WARNING, filename, line_number, SYNTHETIC_ERRNO(EINVAL),
-                                  "Empty %s in \"%s=%s\", ignoring",
-                                  isempty(line + 1) ? "key" : "value",
+                                  "Empty key in \"%s=%s\", ignoring",
                                   line, value);
 
         STRV_FOREACH(entry, match_list)