I am not sure if "There isn't salt" is strictly incorrect, but it does sound strange. You would be advised to replace it with (1) There is no salt All of the others you may use depending on the context that fits it best. (2) There is salt - some salt exists of an indefinite amount (3) There is some salt - same as (2), except perhaps emphasizing that the amount of salt is relatively small (4) There isn't any salt - equivalent to (1)
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