find_language_facts exercise: use dig method

[c-auri]

Because

The current solution to the find_language_facts exercise raises a NoMethodError when trying to access a language_name that is not listed in the languages hash. The lesson mentions the dig method for that specific purpose, but this is never tested in the exercises.

This PR

• Updates the provided solution to use the dig method instead of nested bracket notation.
• Adds a test case with a language_name that does not exist in languages.

Issue

I opened no issue as I figured this is a small enough change to be discussed directly in a PR.

Additional Information

The main branch is updated in #87

Pull Request Requirements

• [x] I have thoroughly read and understand The Odin Project Contributing Guide
• [x] The title of this PR follows the location of change: brief description of change format, e.g. Data types exercise: Update spec files
• [x] The Because section summarizes the reason for this PR
• [x] The This PR section has a bullet point list describing the changes in this PR
• [x] If this PR addresses an open issue, it is linked in the Issue section
• [x] If this PR includes changes that needs to be updated on the solutions branch, I have created another PR (and linked it to this PR).