Check for empty variables before performing REPLACE

[mjcarroll]

🦟 Bug fix

Summary

If the CMAKE_PREFIX_PATH environment variable was empty, the string replace command could fail with (from https://github.com/gazebosim/gz-math/actions/runs/5057393702/jobs/9154388249?pr=482)

  CMake Error at /usr/share/cmake/gz-cmake3/cmake3/GzUtils.cmake:333 (string):
    string sub-command REPLACE requires at least four arguments.
  Call Stack (most recent call first):
    CMakeLists.txt:153 (gz_build_examples)

This moves the function into it's own file (to make it align with rest of the work from #344), and then checks correctly for empty variables before performing the replacement.

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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[ahcorde]

@osrf-jenkins run tests please!