[BUG]: Template string builder returns format specifier when value is `undefined`

[camdecoster]

Description

templateFormatString returns the format specifier used by d3-format when the value being parsed is undefined.

Screenshots/Video

Steps to reproduce

• Load the following mock using plotly.js DevTools:

  {
    "data": [
      {
      "hovertemplate": "Time: %{x}<br>User: %{y}<br>Usage: $%{z:.2f}<extra></extra>",
      "showscale": true,
      "z": {
        "dtype": "f8",
        "bdata": "fDCpKCrK3z8qBd4FpbLBv2heJFDdueQ/K6oBjlRe+D9AplffvPjNvzvbcCYz+M2/CHRAqHRE+T/z1/BG047oP4ueeUveC96/AAAAAAAA+H9wG8Guoqjdv+fvFiuEzt2/cPmepZ74zj++Ne6+y5z+vwAAAAAAAPh/YzyhakL+4b95UaNnjjTwvx+GFtigHNQ/+QOLgYgO7b/HExTEy5j2v0ENnx9Mc/c/2Zhm4TzmzL/GNbFGh0mxP2InDcHEy/a/KPrKUZVr4b81BjE/bGW8PxX1zj14avK/6N19s28L2D/SOz2hbjjjvwAAAAAAAPh/p/2pOS5B479X9hhz7qL9P4aDsNZupIu/AAAAAAAA+H83Lh+1SVLqPxx64lmTiPO/tWnv0gq8yj/DMFAPz1r/v/9asANAQPW/QDr0vL8yyT++nUiqhKHnPwwupFVl78U/XzGyNiCbvb/hr1BvSEXTvwa4sKwGqPe/J6aouPYI578AAAAAAAD4f2LC1/746fA/iEEAkdf91T9OFQ2YaTX8vwAAAAAAAPh/QA8qJjCl2L9NOahTWKnlv/nsCSfakuM/1Pz0Wvl+8D8enOD2C83tP1MVD7Xe2uq/zylztCLK07+KTA+JazPVPwBoGGqqN+8/VWYobMqq3r8AAAAAAAD4fzBuPE2Ms/G/9PWajqkj87+w1UonNgDqP/3Ng74os/U/+iMrXUFvsr9qgA6DeA7wP0KkKG0LJdc/rwoBL9Kk5L+P6aQIGyHXPwAAAAAAAPh/UhmlZMpXor8AAAAAAAD4f19fz+s89QTAv1gZewZN6j8hrkt3t0i2P8Dt4LnvItO/OIpGXqJ9tz/iHOwPFc3/v8FsDls1Hsy/04nor+7a1j8AAAAAAAD4fwAAAAAAAPh/S3Lu+S3f6b+hdpfJZA7gvwAAAAAAAPh/gMUgGEIK1T/02ryry/PgvwNc6tKvbOA/AAAAAAAA+H9TvHjHI//uP3hIyAw4d+a/l5Lxpmr41L8apYzLTBjZvxD0dqaOave/IzBrdqLz0j8UgR0sIbXQPw06yBDZ8XQ/hE70gPMGzr8EJo3KW6X2v3SGH1za69q/CzQW3gjv1b98MTRhQazpv8BzdZwCpcS/Mxz8H/jb2T8bAn5E0S3+P5EnXNaQWMY/AAAAAAAA+H9gVYk14w6zv+1yI3JJs/6/OEbHD3Qmm78AAAAAAAD4f7gI2ke4tANA5IZhukifyL9yMGE5jUzTPwgyV7e9xaG/zxHyveey8r9cb1eTAEnyP1Z/Bd3VD+g/",
        "shape": "10, 12"
      },
      "type": "heatmap"
      }
    ]
  }

• Hover over one of the blank squares
• Note that the format specifier is shown for the usage value

Notes

• This was first noted in plotly/plotly-studio#1504

• The data is being passed in a typed array, which is one method of passing data between plotly.py and plotly.js (this issue was originally found when running a Dash app)

• This happens because NaN values are "cleaned" when processing the input data. NaN gets changed to undefined and a warning message is logged to the console:

  WARN: Variable 'z' in hovertemplate could not be found!