Running diff for resolved issues and html output results in error

[geeksmith]

Describe the bug
When running 'CodeChecker cmd diff --resolved --output html ...' I get this stack trace:

   49m24s CodeChecker    [INFO 2023-05-15 11:01] - Matching local report directories (--newname): /<path>/ccreports
   49m24s CodeChecker    [INFO 2023-05-15 11:01] - Given remote runs (--baseline): baseline_devel
   49m24s CodeChecker    [INFO 2023-05-15 11:01] - Checking local passwords or tokens in /<path>/ccreports/codechecker.passwords.json
   49m24s CodeChecker    [INFO 2023-05-15 11:01] - Checking for local valid sessions.
   54m21s CodeChecker    Traceback (most recent call last):
   54m21s CodeChecker      File "/usr/local/lib/python3.9/dist-packages/codechecker_common/cli.py", line 209, in main
   54m21s CodeChecker        sys.exit(args.func(args))
   54m21s CodeChecker      File "/usr/local/lib/python3.9/dist-packages/codechecker_client/cmd/cmd.py", line 584, in __handle
   54m21s CodeChecker        cmd_line_client.handle_diff_results(args)
   54m21s CodeChecker      File "/usr/local/lib/python3.9/dist-packages/codechecker_client/cmd_line_client.py", line 1292, in handle_diff_results
   54m21s CodeChecker        get_diff_remote_run_local_dir(
   54m21s CodeChecker      File "/usr/local/lib/python3.9/dist-packages/codechecker_client/cmd_line_client.py", line 1024, in get_diff_remote_run_local_dir
   54m21s CodeChecker        filtered_reports.extend(convert_report_data_to_report(results))
   54m21s CodeChecker      File "/usr/local/lib/python3.9/dist-packages/codechecker_client/cmd_line_client.py", line 858, in convert_report_data_to_report
   54m21s CodeChecker        report = report_type_converter.to_report(
   54m21s CodeChecker      File "/usr/local/lib/python3.9/dist-packages/codechecker_client/report_type_converter.py", line 68, in to_report
   54m21s CodeChecker        get_file(report.fileId, report.checkedFile),
   54m21s CodeChecker      File "/usr/local/lib/python3.9/dist-packages/codechecker_client/cmd_line_client.py", line 813, in cached_report_file_lookup
   54m21s CodeChecker        file_cache[file_id].content = convert.from_b64(
   54m21s CodeChecker      File "/usr/local/lib/python3.9/dist-packages/codechecker_web/shared/convert.py", line 22, in from_b64
   54m21s CodeChecker        return base64.b64decode(string_b64.encode("utf-8")).decode(
   54m21s CodeChecker    AttributeError: 'NoneType' object has no attribute 'encode'

CodeChecker version
6.21.0-rc (installed via pip)

To Reproduce

1. Run analysis on project and store results on server.
2. Cherry-pick new commit containing changes, possible fixes, possible new defects.
3. Run diff command to check for new issues: CodeChecker cmd diff --new --uniqueing on --url https://<url> --basename baseline_devel --newname /<path>/ccreports --output html --export-dir /<path>/ccreports/ccdiff-resolved
4. Run diff command to check for resolved issues: CodeChecker cmd diff --resolved --uniqueing on --url https://<url> --basename baseline_devel --newname /<path>/ccreports --output html --export-dir /<path>/ccreports/ccdiff-resolved1
5. Received error above.

Expected behaviour
I expect a report of issues that were resolved in the cherry-picked change.

Desktop (please complete the following information)

• OS: Ubuntu Jammy 20.04
• Browser: N/A
• Version: N/A

Additional context
Seems like there's some entry either locally or in the remote database that has an empty value that CodeChecker isn't expecting.

[geeksmith]

This gets past the error but I'm not sure what side effects it may introduce:

diff --git a/web/client/codechecker_client/cmd_line_client.py b/web/client/codechecker_client/cmd_line_client.py
index f73b3af2..7a51ea1b 100644
--- a/web/client/codechecker_client/cmd_line_client.py
+++ b/web/client/codechecker_client/cmd_line_client.py
@@ -787,7 +787,7 @@ def handle_diff_results(args):
                 source = client.getSourceFileData(
                     file_id, True, ttypes.Encoding.BASE64)
                 file_cache[file_id].content = convert.from_b64(
-                    source.fileContent)
+                    source.fileContent) if hasattr(source.fileContent, "encode") else source.fileContent
 
         return file_cache[file_id]

[geeksmith]

Passing '--uniqueing off' avoids this error without any changes to source code.