Fix null check error and improve error handling in web implementation

[BilalRehman08]

This PR addresses the issue #761 where a "Null check operator used on a null value" error occurs when accessing the storage through an IP address on a local network.

Changes made:

• Modified the read method in FlutterSecureStorageWeb to handle potential null values more safely
• Added null check before attempting to decrypt values
• Implemented error handling for the decryption process