Fix empty case for metadata selectors & fix invalid metadata case
Fix empty case for metadata selectors & fix invalid metadata case

rlugojr/cytoscape.js,cytoscape/cytoscape.js,KonstantinShemyak/cytoscape.js,cytoscape/cytoscape.js,soomin-maeng/cytoscape.js,KonstantinShemyak/cytoscape.js,soomin-maeng/cytoscape.js,rlugojr/cytoscape.js