python: avoid empty LD_LIBRARY_PATH/PATH entries on import (4.x backport of #29905)

Backport of #29905 to the 4.x branch, applied verbatim: after this change
`modules/python/package/cv2/__init__.py` is byte-identical to the 5.x version
(same blob 99685b3791), since both branches carried the same code here.

`import cv2` unconditionally rewrites the environment; when LD_LIBRARY_PATH
was previously unset this creates a dangling separator, i.e. an empty entry
which, per ld.so(8), resolves to the current working directory of every
subsequently spawned child process. Children may then pick up same-named
shared libraries from the CWD and fail (real-world case reported in
opencv/opencv-python#1268: a Nuitka-standalone app's `xdg-open -> kde-open`
dying with `libssl.so.3: version 'OPENSSL_3.2.0' not found`; reproduced on
4.11.0.86 - 4.14.0.94 manylinux wheels). The Windows PATH line has the same
dangling-separator pattern.

Identical to #29905: factor the prepend into _prepend_env_paths(), filter
empty entries, write the variable only when there is something to add, and
join with the old value without producing an empty entry.

The follow-up question of not mutating the process environment at all
remains tracked in #28994.

modules/python/package/cv2/__init__.py

@@ -20,6 +20,14 @@ except ImportError:
 # is_x64 = sys.maxsize > 2**32
 
 
+def _prepend_env_paths(env_key, paths, sep):
+    extra = [p for p in paths if p]
+    if not extra:
+        return
+    old = os.environ.get(env_key)
+    os.environ[env_key] = sep.join(extra) + ((sep + old) if old else '')
+
+
 def __load_extra_py_code_for_module(base, name, enable_debug_print=False):
     module_name = "{}.{}".format(__name__, name)
     export_module_name = "{}.{}".format(base, name)
@@ -140,11 +148,11 @@ def bootstrap():
                 except Exception as e:
                     if DEBUG: print('Failed os.add_dll_directory(): '+ str(e))
                     pass
-        os.environ['PATH'] = ';'.join(l_vars['BINARIES_PATHS']) + ';' + os.environ.get('PATH', '')
-        if DEBUG: print('OpenCV loader: PATH={}'.format(str(os.environ['PATH'])))
+        _prepend_env_paths('PATH', l_vars['BINARIES_PATHS'], ';')
+        if DEBUG: print('OpenCV loader: PATH={}'.format(str(os.environ.get('PATH'))))
     else:
         # amending of LD_LIBRARY_PATH works for sub-processes only
-        os.environ['LD_LIBRARY_PATH'] = ':'.join(l_vars['BINARIES_PATHS']) + ':' + os.environ.get('LD_LIBRARY_PATH', '')
+        _prepend_env_paths('LD_LIBRARY_PATH', l_vars['BINARIES_PATHS'], ':')
 
     if DEBUG: print("Relink everything from native cv2 module to cv2 package")