Avoid temp dir creation at import time for DefaultOptions

[sfmqrb]

Summary

Fixes #177 by making temporary DB directory creation lazy.

Problem

DefaultOptions currently calls tempDBDir() during package initialization, which creates a new temp directory even if the caller never opens a DB.

Changes

• DefaultOptions.DirPath is now initialized to an empty string.
• validateOptions now lazily assigns tempDBDir() when DirPath is empty.
• Updated TestDBOpen to verify that opening with empty DirPath succeeds and gets a generated directory.

Behavior after this PR

• Importing lotusdb no longer creates temp directories.
• A fresh temp dir is still automatically created when Open is called with no DirPath.

Validation

• go test ./... passes locally in this environment.