fix: treat empty on-disk JSON state files as empty state

[mwbrooks]

Problem

This pull request helps to address unable_to_parse_json internal errors that some users are experiencing.

Root cause hypothesis

afero.WriteFile opens with O_TRUNC before writing. If the process is interrupted (SIGKILL, host crash, oom, user cancel) after the truncate but before the write completes, the JSON state file is left at zero bytes. Every subsequent CLI invocation then reads the file, calls json.Unmarshal(nil, …) on the empty contents, and raises ErrUnableToParseJSON. The file never self-heals: the reader errors out before the caller reaches the code path that would rewrite it.

Affected state files:

• ~/.slack/credentials.json
• ~/.slack/config.json
• Project .slack/config.json
• Project .slack/apps.json
• Project .slack/apps.dev.json

What this PR changes

Adds a single helper goutils.IsEmptyJSON([]byte) bool and updates the five readers so that an empty (or whitespace-only) file returns the same zero-value state as the "file does not exist" branch, rather than surfacing a parse error:

• internal/auth/auth.go — auths() returns an empty AuthByTeamID
• internal/config/system.go — UserConfig returns a config with Surveys initialised
• internal/config/project.go — ReadProjectConfigFile returns a config with Surveys initialised
• internal/app/app_client.go — readDeployedApps and readLocalApps return empty maps and re-serialise a valid {} so the file self-heals on the next write cycle

Genuinely malformed JSON still returns ErrUnableToParseJSON. Existing broken-JSON tests are untouched.

Test plan

• Test_IsEmptyJSON covers empty, whitespace-only, {}, and populated inputs
• Test_Client_auths empty-file subtest returns no auths, no error
• Test_SystemConfig_UserConfig empty-file subtest returns zero-valued config, no error
• Test_ReadProjectConfigFile empty-file subtest returns zero-valued config, no error
• AppClient.readDeployedApps empty-file subtest returns empty apps and writes {} back to disk
• AppClient.readLocalApps empty-file subtest returns empty local apps and writes {} back to disk
• go test ./internal/... ./cmd/... passes
• make lint passes with zero issues

Follow-ups (not in this PR)

• Atomic writes — replace afero.WriteFile with a temp-file + Rename pattern across the five state writers so the durable fix is that these files can never reach a 0-byte state in the first place.
• Centralise the read-JSON-state pattern — the "stat → read → check empty → unmarshal → wrap error" sequence is now duplicated across five call sites. Move it into goutils (or a new internal/statefile package) so future state files pick up the guard for free.
• internal/api/activity.go — ErrHTTPResponseInvalid on the activity poll loop shows the same "thousands of events off one broken response" shape; worth a matching guard.
• internal/update/sdk.go — retry with backoff on persistently malformed SDK hook response so a broken SDK doesn't dominate the error telemetry either.

[codecov]

Codecov Report

❌ Patch coverage is 81.81818% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 71.74%. Comparing base (a265b95) to head (77804be).
⚠️ Report is 4 commits behind head on main.

Files with missing lines	Patch %	Lines
internal/app/app_client.go	66.66%	2 Missing and 2 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #616      +/-   ##
==========================================
+ Coverage   71.72%   71.74%   +0.02%     
==========================================
  Files         226      226              
  Lines       19182    19204      +22     
==========================================
+ Hits        13758    13778      +20     
  Misses       4214     4214              
- Partials     1210     1212       +2     

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