fix: only pass non-nil span options

[mweibel]

fixes #37

[jqueuniet]

Hey, thanks for the patch, I tried it out and it does solve the panic issue, but I also noticed you used an options variable name which shadows an internal options type from this project (defined in options.go line 25). It would probably be for the best to change this variable name and avoid the naming conflict, even if it does not seem to cause any actual issue.

[mweibel]

@jqueuniet ah yeah, good catch. Fixed that.