Skip empty console messages in JSON output mode

[Copilot]

azd auth token -ojson emits a spurious {"type":"consoleMessage","data":{"message":"\n"}} line before the error JSON when authentication fails. This is caused by console.Message(ctx, "") calls used as visual separators (blank lines) in text mode — in JSON mode, EventForMessage("") appends a trailing \n to the empty string, producing a meaningless event.

Changes

• cli/azd/pkg/input/console.go — Early-return in Message() when the message is empty and format is JSON. Empty messages are visual separators with no semantic value in structured output.
• cli/azd/pkg/input/console_test.go — Added TestAskerConsole_Message_EmptySkippedInJson covering both the skip and the pass-through for non-empty messages.

Before:

$ azd auth token -ojson --tenant-id test
{"type":"consoleMessage","timestamp":"...","data":{"message":"\n"}}
{"error":"fetching token: ...","message":"Authentication with Azure failed.","suggestion":"..."}

After:

$ azd auth token -ojson --tenant-id test
{"error":"fetching token: ...","message":"Authentication with Azure failed.","suggestion":"..."}