Updates to maintain compat with Tornado >= 4.5
Updates to maintain compat with Tornado >= 4.5

issues: fixes #6152

Explicitly check for None and do not rely on BadYieldError being raised

ericmjl/bokeh,mindriot101/bokeh,timsnyder/bokeh,philippjfr/bokeh,stonebig/bokeh,Karel-van-de-Plassche/bokeh,bokeh/bokeh,DuCorey/bokeh,DuCorey/bokeh,schoolie/bokeh,aavanian/bokeh,ericmjl/bokeh,schoolie/bokeh,Karel-van-de-Plassche/bokeh,stonebig/bokeh,percyfal/bokeh,stonebig/bokeh,percyfal/bokeh,philippjfr/bokeh,ericmjl/bokeh,philippjfr/bokeh,dennisobrien/bokeh,schoolie/bokeh,rs2/bokeh,aavanian/bokeh,dennisobrien/bokeh,aavanian/bokeh,mindriot101/bokeh,timsnyder/bokeh,mindriot101/bokeh,timsnyder/bokeh,rs2/bokeh,bokeh/bokeh,ericmjl/bokeh,DuCorey/bokeh,jakirkham/bokeh,jakirkham/bokeh,dennisobrien/bokeh,jakirkham/bokeh,philippjfr/bokeh,bokeh/bokeh,percyfal/bokeh,dennisobrien/bokeh,timsnyder/bokeh,bokeh/bokeh,Karel-van-de-Plassche/bokeh,bokeh/bokeh,percyfal/bokeh,jakirkham/bokeh,percyfal/bokeh,DuCorey/bokeh,aavanian/bokeh,schoolie/bokeh,jakirkham/bokeh,DuCorey/bokeh,rs2/bokeh,Karel-van-de-Plassche/bokeh,ericmjl/bokeh,timsnyder/bokeh,Karel-van-de-Plassche/bokeh,rs2/bokeh,philippjfr/bokeh,dennisobrien/bokeh,rs2/bokeh,aavanian/bokeh,stonebig/bokeh,mindriot101/bokeh,schoolie/bokeh