texttemplate: accept empty text string

[antoinerg]

This closes #4173 which affected the following traces:

• pie
• funnelarea
• sunburst

Commit 9b4a38d introduces a test to 🔒 and find all the traces with the bug.

Commit 2e966e0 contains the fix

Codepen before
Codepen after

[archmoj]

Looking good to me.

[archmoj]

@antoinerg Thanks for the quick PR which is going to be helpful for our treemap too.
I would let @etpinard review it & give you a dancer or two : )

[antoinerg]

Thanks! Would we mind adding a test for aggregated pie + texttemplate

Done in a4bce50

[etpinard]

Great fix! Thanks 💃