fix no-element exception when href is absent

[arthurbcd]

Description:

Issue: Currently, the GPX reader crashes when it encounters a GPX link () element without an href attribute.

Proposed Solution: This PR introduces a safeguard against such scenarios. Instead of crashing, the reader will now handle missing href attributes gracefully.

Implementation Details:

Added a check for the presence of the href attribute in a GPX link.

• If href is present, the reader behaves as usual, extracting its first value.
• If href is absent, the reader will not attempt to access it, avoiding a crash. This is seamless as our default href parameter is already set to an empty string ('').

Benefit: This change enhances the robustness of the GPX reader, allowing it to process GPX files with missing href attributes in links without failing.