fix: start the LaunchDarkly client offline when LD_SDK_KEY is unset

[devin-ai-integration]

Summary

Running the example without LD_SDK_KEY crashed the whole application with a multi-thousand-line supervisor stack trace, because os:getenv/1 returns the atom false (not "") when the variable is unset and that atom was passed straight into ldclient:start_instance/2.

• hello_erlang_server:init/1 now dispatches through start_ldclient/1, which starts the client with #{offline => true} and logs a warning when the key is missing or empty.
• Evaluations then return their fallback value instead of the gen_server dying with noproc on the first hello_erlang_server:get/3 call.
• No dependency changes: ldclient 3.11.2 is the latest release in the current major, and rebar3 upgrade --all reports every locked transitive dependency already at its latest resolvable version.
• No deprecated SDK APIs and no deprecation warnings when compiling or running.

Implementation details

start_ldclient(SdkKey) when SdkKey =:= false; SdkKey =:= "" ->
  logger:warning("LD_SDK_KEY is not set; ..."),
  ldclient:start_instance("", #{offline => true});
start_ldclient(SdkKey) ->
  ldclient:start_instance(SdkKey, #{http_options => #{tls_options => ldclient_config:tls_basic_options()}}).

Skipping start_instance entirely was rejected: ldclient:variation/3 then exits, so the example would still fail on the first evaluation.

Testing (Erlang/OTP 27 via docker run erlang:27, rebar3 compile + rebar3 shell):

• With a real server SDK key: streaming connection established, hello_erlang_server:get(<<"hello-boolean">>, false, <<"user-key-123abc">>) returned the flag value from LaunchDarkly.
• With LD_SDK_KEY unset: warning logged, application boots, same call returns the false fallback (previously: crash).

Not a UI change, so no screenshots or staging link apply.

Link to Devin session: https://app.devin.ai/sessions/39c567ec09c14cb3973cb2f73337f733
Open in Devin Desktop: https://app.devin.ai/desktop/session/39c567ec09c14cb3973cb2f73337f733?variant=devin

[devin-ai-integration]

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[devin-ai-integration]

@cursor review