Lack of `fi` resulted in a syntax error.
Also, change `;` to `||` to ensure it builds successfully on architectures other than `linux/arm/v7`.
```
[2023-10-24T10:46:49.941Z] ------
> [dev_container_auto_added_stage_label 11/18] RUN mkdir -p "/home/archivebox/.config/chromium/Crash Reports/pending/" && chown -R archivebox "/home/archivebox/.config" ; if [[ "linux/amd64" == "linux/arm/v7" ]]; then $exit 0; else exit 1:
0.401 /bin/bash: -c: line 2: syntax error: unexpected end of file
------
WARNING: buildx: git was not found in the system. Current commit information was not captured by the build
[2023-10-24T10:46:49.942Z] Dockerfile-with-features:135
--------------------
134 | && ln -s "$CHROME_BINARY" /usr/bin/chromium-browser
135 | >>> RUN mkdir -p "/home/${ARCHIVEBOX_USER}/.config/chromium/Crash Reports/pending/" \
136 | >>> && chown -R $ARCHIVEBOX_USER "/home/${ARCHIVEBOX_USER}/.config" \
137 | >>> ; if [[ "$TARGETPLATFORM" == "linux/arm/v7" ]]; then $exit 0; else exit 1
138 | # ignore failure for architectures where no playwright release is available yet
--------------------
ERROR: failed to solve: process "/bin/bash -c mkdir -p \"/home/${ARCHIVEBOX_USER}/.config/chromium/Crash Reports/pending/\" && chown -R $ARCHIVEBOX_USER \"/home/${ARCHIVEBOX_USER}/.config\" ; if [[ \"$TARGETPLATFORM\" == \"linux/arm/v7\" ]]; then $exit 0; else exit 1" did not complete successfully: exit code: 2
```
This slightly modifies the HTML_TITLE_REGEX to fix two parsing errors.
The first occurred when title tags were empty (e.g. "<title></title>")
which was parsed as "</title". The second occurred when titles were a
single character (e.g. "<title>A</title>") which was not matched by the
regex, and so would fall back to link.base_url.
Now when tags are empty, it falls back to link.base_url, and single
character titles are parsed correctly.
The way the regex works now is still a bit wonky for some edge cases.
I couldn't find any cases of incorrect behavior, but it still might be
worth reworking more completely for robustness.
