[Maps] Tile Layer Attibution text and Attribution link validation errors

[nickpeihl]

Kibana version:
7.9.0-snapshot

Describe the bug:
When adding a custom tile layer to Maps, the Attribution text and Attribution link text fields show validation errors even when the fields are filled out correctly.

Steps to reproduce:

1. Add a Tile Layer in Maps using https://services.arcgisonline.com/arcgis/rest/services/World_Topo_Map/MapServer/WMTS/tile/1.0.0/World_Topo_Map/default/GoogleMapsCompatible/{z}/{y}/{x}.jpg
2. Enter any text in the Attribution text field
3. Enter any valid URL in the Attribution link field
4. Note that the fields show validation errors

Expected behavior:
Validation errors should only appear if one or the other attribution fields is incomplete.

Screenshots (if relevant):

[elasticmachine]

Pinging @elastic/kibana-gis (Team:Geo)