Drop empty value for php-fpm environment variable

Summary:
php-fpm doesn't accept empty values as environment variables.

It somewhat makes sense, as "" is actually the value
for an undefined environment variable.

Fixes the following issue:

[27-Feb-2020 12:00:23] ERROR: [/usr/local/etc/php-fpm-env.conf:9] empty value
[27-Feb-2020 12:00:23] ERROR: Unable to include /usr/local/etc/php-fpm-env.conf from /usr/local/etc/php-fpm.conf at line 9
[27-Feb-2020 12:00:23] ERROR: failed to load configuration file '/usr/local/etc/php-fpm.conf'
[27-Feb-2020 12:00:23] ERROR: FPM initialization failed

Ref T1598.

Test Plan: Run container and ensure php-fpm initializes

Reviewers: dereckson

Reviewed By: dereckson

Maniphest Tasks: T1598

Differential Revision: https://devcentral.nasqueron.org/D2267