Fix isRootRelative for protocol-relative URLs

What does this MR do?

Fix isRootRelative for protocol-relative URLs

Protocol-relative URLs, e.g., //foo.test, are not root-relative. This fixes isRootRelative to return false when given a protocol-relative URL using a negative lookahead assertion for a second '/' character.

Screenshots (strongly suggested)

n/a

Does this MR meet the acceptance criteria?

Conformity

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Availability and Testing

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