Create trace is failing when there's no description

[HarelM]

Bug/Feature

Create trace is failing when there's no description with a cryptic error message: cannot read value of null, parameter content, or something similar.

This is caused by an error returning from OSM API call to create the trace. There should be better error handling. @blackboxlogic let me know how you'd like to proceed with this.

[blackboxlogic]

According to the docs all 5 parameters are required. Two options:

• Add client side validation (probably to all 5 gpx properties referenced in CreateTrace()) and throw a descriptive ArgumentException. I tried to consolidate validation in validate.cs.
• Use some default value like empty string. (Currently how the trace's Tags are handled if null, but I didn't have a good reason for doing that)
• Some third thing I haven't thought of?

I have a slight preference for default values but you're more of a steakholder than I am for this functionality. I don't know if you are a contributor to this repo (I don't have permission to see that). If you are, feel free to make a change. If you're not, let me know which option you'd like and I can do it.

[HarelM]

I do have permissions to commit. I'll see if I can add the relevant validation to validate.cs

[HarelM]

@blackboxlogic feel free to review my changes.