fix tooltip bug: child nodes without tooltips were showing the nearest parent tooltip.
fix tooltip bug: child nodes without tooltips were showing the nearest parent tooltip.

refs #107

citation-style-editor/csl-editor,adam3smith/csl-editor,fbennett/csl-editor,adam3smith/csl-editor,citation-style-editor/csl-editor,citation-style-language/csl-editor,citation-style-language/csl-editor,fbennett/csl-editor,citation-style-editor/csl-editor,citation-style-language/csl-editor,adam3smith/csl-editor