fix(BpmnSemantic): do not display properties with undefined values

[tbouffard]

Except for the "name" property (found in the header), only valued properties are displayed, so that only data of interest is shown.

closes #549

Live environment of examples

https://cdn.statically.io/gh/process-analytics/bpmn-visualization-examples/fix/549-hide_undefined_properties_of_BpmnSemantic/examples/custom-behavior/javascript-tooltip-and-popover/index.html