GUI option for startOffset not updated if attribute is undefined

[su_v]

Follow-up report to
* Bug #171005 “Arbitrary position for a text on a path”
* Bug #171033 “GUI for startOffset attribute of text path”

The GUI option in the Text and Font dialog for the attribute 'startOffset' of a <textPath/> element is not updated as expected if the attribute is undefined (the default for text put on path in Inskcape). This can cause confusion if an earlier selected text put on path was positioned e.g. at 50% - the old value is not cleared when selecting a different text-put-on-path which starts at offset 0 (omitted attribute).

Steps to reproduce:
1) launch inskcape
2) write a short text
3) draw a circle
4) select all, duplicate and move side-by-side
5) format second text to be center-aligned
   (without highlighting any text content, to avoid triggering bug #799736)
6) put each text on a circle
7) deselect all, open 'Text and Font' dialog
8) select the first text
--> Text path offset: 0%
9) select the second text
--> Text path offset: 50%
10) select the first text again

Expected result:
Text path offset: 0%

Actual result:
Text path offset: 50% (unchanged from step 9)

11) with the first text still selected, change its offset e.g. to 25%
12) repeat steps 8-10:
--> expected results (correct updates of the 'Text path offset' value)

Reproduced with Inkscape 0.91, 0.92.0, 0.92.1 on OS X 10.7.5 and Windows 10.