[nad-viewer] Bus legend displayed even if empty

[flo-dup]

Describe the current behavior

When generating voltage level legend the generated bus legends are kept even if all the strings are empty, leading to an empty bus legend (lines with only the colored square and no text)

Describe the expected behavior

When generating voltage level legend the bus legends are added only if the string is not empty

Describe the steps

• Create a nad with the DefaultLabelProvider and a LabelProviderParameters with isBusLegend set to false.
• Launch the nad-viewer with that diagram and enableAdaptiveTextZoom = true
• Zoom into a voltage level to see the generated legend

Environment

No response

Relevant Log Output

No response

Extra Information

No response