ren: end-of-sentence spacing when followed by font/size changes

The adj_nonl() command was added to prevent newlines for lines
containing only commands that do not print anything (like font or size
changes).  This patch makes sure render() does not call adj_nonl()
unnecessarily; otherwise, it may inhibit the insertion of a space
after end-of-sentence characters (when followed by a line break
starting with \f escape sequence).