filterdiff --add[|old|new]prefix should not add prefixes to /dev/null

[peat-psuwit]

Steps to reproduce: run the following command:

curl --location https://github.com/qt/qtwebengine-chromium/commit/693211aa99fb3fe45547d6833417fa9badda385c.patch | \
    ./src/filterdiff --strip=1 --addprefix=src/3rdparty/

And look at the last changed file, which is actually adding file.

Expected result:

--- /dev/null
+++ src/3rdparty/chromium/third_party/blink/public/strings/blink_strings_grd/IDS_MEDIA_SESSION_DATA_SOURCE.png.sha1
@@ -0,0 +1 @@
+55ed99b05a2a64d5a0552b39828c15b688536eca
\ No newline at end of file

Actual result:

--- src/3rdparty//dev/null
+++ src/3rdparty/chromium/third_party/blink/public/strings/blink_strings_grd/IDS_MEDIA_SESSION_DATA_SOURCE.png.sha1
@@ -0,0 +1 @@
+55ed99b05a2a64d5a0552b39828c15b688536eca
\ No newline at end of file

This is patchutils Git commit b5a6f19 on Ubuntu 26.04.