Make JEXL and CSEL behave the same for expressions without a leading slash

NEXUS-16703 made CSEL and JEXL content selectors work the same for expressions with a leading slash. The book and support documentation all push users towards using a leading slash in their path expressions.

However there is still a discrepancy if you create a content selector with a path expression that doesn't have a leading slash. It will match in CSEL because internally the asset path doesn't have a leading slash, but it won't match in JEXL because it prefixes the path with a slash when creating the JXEL model (this was done previously to get JEXL to work with expressions that have a leading slash). Note that this also happens on master before NEXUS-16703, so this is not a recent regression.

Also note that expressions migrated over from NXRM 2 should have leading slashes because that was the only supported approach. So this story is specifically dealing with expressions created in NXRM 3 that go against the advice to use a leading slash.

The workaround is simple: introduce a leading slash as shown in the book examples.