Fix empty updates in dynamic init scripts

[dra27]

An update of the form:

FOO += ""

is not supposed to do anything to the environment (#5350). However, these updates were being written to the init scripts in opam-init (variables.sh, etc.). This PR fixes that, and consequently the problem with mingw-w64-shims reported in dra27/mingw-w64-shims#2 and #6053 (comment).

Before this PR, for a switch with mingw-w64-shims and only the x86_64 installation, variables.sh has:

# Updated by package mingw-w64-shims
PATH='C:\Devel\Roots\test-empty\.cygwin\root\usr\x86_64-w64-mingw32\sys-root\mingw\bin':"$PATH"; export PATH;
# Updated by package mingw-w64-shims
PATH='':"$PATH"; export PATH;

which is wrong, but after just:

# Updated by package mingw-w64-shims
PATH='C:\Devel\Roots\test-empty\.cygwin\root\usr\x86_64-w64-mingw32\sys-root\mingw\bin':"$PATH"; export PATH;

Both before and after, the switch's environment file correctly has both entries:

PATH    +=      C:\\Devel\\Roots\\test-empty\\.cygwin\\root\\usr\\x86_64-w64-mingw32\\sys-root\\mingw\\bin      Updated\ by\ package\ mingw-w64-shims
PATH    +=      @       Updated\ by\ package\ mingw-w64-shims

[kit-ty-kate]

modulo the fixup commit i've just pushed (feel free to remove it or squash it if you disagree or agree with it), that looks ok to me, although i'm not an expert in this part of the code.

Could you add a test in the reftests to avoid future regressions? From afar it looks reasonably easy to craft a test for this particular case