[9.x] Proper fix when provided path is '0'

[vlakoff]

#40013 was incorrect, because when provided argument was '0',
we were appending $path (i.e. '0') instead of DIRECTORY_SEPARATOR.$path (i.e. '/0').

To summarize the behavior after this PR:

argument	appended string
'foo'	'/foo'
'0'	'/0'
'' (default)	''

In addition, as I'm using a loose comparison, it supports the following cases (they are not supported nor expected, though it doesn't harm to work better with them):

argument	appended string
null	''
false	''

[4m1n0s]

i think this fix must be applied to other functions too such as: path, basePath, bootstrapPath, configPath.....

[vlakoff]

As a reminder, I used $path != '' which is equivalent to (string) $path !== ''.
I choosed the former because it is shorter, but the latter might be preferable so people are not inclined to think that $path != '' should be replaced with with $path !== ''.