When resolving resources, the PathResourceResolver creates a Resource
instance and checks whether this resource `exists()` and `isReadable()`.
While that last call returns false for folders on the file system, both
calls return true for folders located inside JARs.
If a JAR location is configured as a resource location, then
PathResourceResolver can resolve folders in JARs as valid locations and
candidates for paths resolution.
Prior to this change, the PathResourceResolver would resolve "" as a
valid resource path (here, the "/META-INF/resources/webjars" if
configured, for example) and return a "" path for this resource,
effectively turning all "/" URLs into empty ones "".
This commit fixes the resolveUrlPathInternal implementation by not
allowing empty paths as valid resource paths.
Issue: SPR-13241
daf94ea576