cgeo: Export of calculated WP to PN not working

@murggel I tried to test PR #10849 as follows:

• Took a traditional cache
• Added a reference point with calculated coordinates as follows: N51 AB.139 E007 CD.529 A=t B=f C=u D=r f=3 r=4 t=5 u=2

This results in N51 53.139 E007 24.529

• If I backup the waypoints to the PN it adds only:

{c:geo-start}
@Markierung 1 (W)
{c:geo-end}

It seems not working even not if I ommit some values for variable to have an incomplete formula.

Tested with (to be release) 2021.06.06 as well as current NB.