Fix undefined WPR_WEBP env var

[dvershinin]

Improves readability of .htaccess by preventing the inclusion of undefined WPR_WEBP environment variable when WebP caching is disabled.

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• My changes generate no new warnings
• [x ] New and existing unit tests pass locally with my changes

[dvershinin]

@Roy-Orbison updated with the suggestion and squashed it into a single commit.

[dvershinin]

@Roy-Orbison can we have this merged now? Thank you in advance :-)

[Roy-Orbison]

@dvershinin I don't work for them, sorry. I just read your commit when I was doing one myself (#6123).

[dvershinin]

Oh, I see. @engahmeds3ed would you take a look, please? We're working on a converter for Apache configurations, and this is one of the issues somewhat critical for us.