Fixed release script breaking if there is no previous version

This can't actually happen in the current state of the littlefs GitHub
repo, but could in theory cause problems if CI is enabled on a fork.

Found while enabling GitHub Actions on littlefs-fuse.

.github/workflows/release.yml

@@ -81,7 +81,9 @@ jobs:
       - name: find-prev-version
         continue-on-error: true
         run: |
-          LFS_PREV_VERSION="$(git describe --tags --abbrev=0 --match 'v*')"
+          LFS_PREV_VERSION="$( \
+            git describe --tags --abbrev=0 --match 'v*' \
+            || true)"
           echo "LFS_PREV_VERSION=$LFS_PREV_VERSION"
           echo "LFS_PREV_VERSION=$LFS_PREV_VERSION" >> $GITHUB_ENV
 
@@ -240,6 +242,7 @@ jobs:
         run: |
           # create release and patch version tag (vN.N.N)
           # only draft if not a patch release
+          touch release.txt
           [ -e table.txt ] && cat table.txt >> release.txt
           echo >> release.txt
           [ -e changes.txt ] && cat changes.txt >> release.txt