Fix finding ogre 2.3 installed from source

[iche033]

🦟 Bug fix

Summary

When building gz with ogre-next 2.3 from source, cmake has trouble finding ogre-next because the OGRE2_LIBRARY_DIRS is empty and OGRE2_RESOURCE_PATH points to the incorrect dir.

• OGRE2_LIBRARY_DIRS: Turns out that variable is set by a STRING REPLACE call that never gets called because the if check before string replace seems incorrect - I updated to make sure the input string exists before doing the replace call (originally the logic checks that the input string does not exist).
• OGRE2_RESOURCE_PATH: when built from source, the ogre plugins are installed to a dir called OGRE-Next instead of OGRE

Tested with ogre-next 2.3 installed from https://github.com/gazebo-forks/ogre-next/tree/gazebo/v2-3

Checklist

• Signed all commits for DCO
• Added tests
• Updated documentation (as needed)
• Updated migration guide (as needed)
• Consider updating Python bindings (if the library has them)
• codecheck passed (See contributing)
• All tests passed (See test coverage)
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Note to maintainers: Remember to use Squash-Merge and edit the commit message to match the pull request summary while retaining Signed-off-by messages.

[mjcarroll]

I think I addressed this in #302, though it is a larger change

[iche033]

I think I addressed this in #302, though it is a larger change

ah I see, I'll give that a try

[j-rivero]

@osrf-jenkins run tests