SVG output generating invalid SVG when using `  `, e.g., in`\liminf`

First reported at https://github.com/mathjax/MathJax-node/issues/6:

• Converting TeX input \liminf to SVG will contain<text transform="scale(53.81925) matrix(1 0 0 -1 0 0)" stroke="none" font-weight="" font-style="" font-family="STIXGeneral,'Arial Unicode MS',serif"> </text> which is invalid since font-weight and font-style are empty.
• Removing the empty attributes fixes this.

IIUC, the problem is that converting TeX input \liminf to MathML produces u+2006 which is not covered by the MathJax TeX fonts (at least according to the error message of the SVG output) but also (in the output) not converted into characters that are covered. In turn, this unknown characters is causing the text tag.

E.g., $a   b$ also produces the problem.