[BUG] invalidNextUrl when the value for nextUrl is the basePath (without trailing forward slash)

[cwperks]

What is the bug?

In #2048, the nextUrl validation logic was updated to account for serverBasePath. There was a miss in the logic that didn't take into account when the value for nextUrl was the basePath only. i.e. if basePath is /osd and nextUrl=%2Fosd the dashboards would give back an Invalid nextUrl parameter. error. The workaround is to add a trailing forward slash like nextUrl=%2Fosd%2F

What is the expected behavior?

If nextUrl is equal to the basePath then it should the nextUrl should be valid.

What is your host/environment?

• OpenSearch 2.16.0