If you have inventory from suppliers and some values are NULL then just use this quick method to display the message you want for those to match 0 inventory levels.

In your Products > Inventory > Settings screen, click to add a New Rule. Select your supplier and other criteria as required and enter messages to display on your storefront.
Ensure that 'Include products without inventory' toggle is enabled and any others required. Hit save and you're all set, any products without stock or 0 stock will display the same message.

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