[os] fix #10017 regression, fix #10025 regression

[timotheecour]

/cc @Araq @krux02 @dom96

• fix regression: normalizePath("foo/..") now incorrectly returns "", should be "." like before + in almost all other languages #10017 regression
• fix [regression] [os] joinPath("usr/", "/lib")="usr/lib" but joinPath(".", "/lib")="/lib" #10025 regression

[Araq]

But then what does "" mean? And why does "" not compare equal to "."?

[timotheecour]

But then what does "" mean? And why does "" not compare equal to "."?

"" means un-initialized. It should never ever mean "current directory"

try:

ls ""
ls: cannot access '': No such file or directory

it will be a source of endless, possibly dangerous, bugs otherwise.
see also @krux02 's comment #10017 (comment) and the fact that pretty much all languages I use (D, python, C++, node JS, go) don't do that mistake, as I explained here #10017 (comment) (including Nim up till that recent regression)

yet another example: in python:

os.path.exists('.')
True
os.path.exists('')
False

liekwise with D:

rdmd --eval 'writeln(exists(""));'
false
rdmd --eval 'writeln(exists("."));'
true

etc...

[timotheecour]

/cc @Araq @krux02 @dom96

2 birds, one stone! latest commit also fixes #10025

[Araq]

Ok, sometimes it's easy to convince me. Thanks a lot!