font-size attribute saved without a value

[RazrFalcon]

When creating a text node with two superscript "items" the last one will be saved without a font-size value.

Simplified result:
<text
       xml:space="preserve"
       style="font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;font-size:10.58333302px;line-height:125%;font-family:'DejaVu Sans';-inkscape-font-specification:'DejaVu Sans';letter-spacing:0px;word-spacing:0px;fill:#000000;fill-opacity:1;stroke:none;stroke-width:0.26458332px;stroke-linecap:butt;stroke-linejoin:miter;stroke-opacity:1"
       x="49.877354"
       y="136.47275"
       id="text4269"><tspan
         sodipodi:role="line"
         id="tspan4267"
         x="49.877354"
         y="136.47275"
         style="stroke-width:0.26458332px">qwe<tspan
   style="font-size:17.19791591%;baseline-shift:super"
   id="tspan4271">qwe<tspan
   style="font-size:;baseline-shift:baseline"
   id="tspan4273">qwe</tspan></tspan></tspan></text>

As you can see it contains "font-size:;baseline".

Steps to reproduce:
1. Create a new file.
2. Write some text.
3. Select a part of the text and make it superscript.
4. Select a part of the superscripted text and make it superscript again.
5. Save the file.

Tested on Inkscape 0.92+devel 15686, Gentoo Linux x86_64

[Patrick Storz (ede123)]

Might be related to bug #1658029.