Added support of empty strings

[dimitriyremerov]

• Added support of empty environment strings
• Added a test case

Note that empty environment strings are not the same as non-set environment strings (~):

$ MEM_DATA= php -a
Interactive shell

php > var_dump(getenv('MEM_DATA'));
string(0) ""
php > ^D

$ php -a
Interactive shell

php > var_dump(getenv('MEM_DATA'));
bool(false)