Your present command doesn’t specify any input, so use
phantomjs runner.js | ffmpeg -f image2pipe -i pipe:.png -c:a copy -c:v libx264 -f flv rtmp://localhost/mystream
There’s no audio input, so setting an audio codec is pointless. If your output needs an audio stream, use
phantomjs runner.js | ffmpeg -f image2pipe -i pipe:.png -f lavfi -i anullsrc -c:v libx264 -c:a aac -f flv rtmp://localhost/mystream