When using igGrid in editmode , the igNumericEditor is used and some editorprovider options can be used. Even when the allowNullValue is specified, according to a support case, "igGrid overrides it to avoid having everyone to set "allowNullValue" to true all the time". The fix is to implement a custom numeric editor provider: https://www.igniteui.com/help/implementing-custom-editor-provider. This is quit overkill just to use a property that is already accessible.
Combining this setting with a minValue and a knockout implementation can make a lot of sense and should be supported.
See this jsfiddle for a simplified sample: http://jsfiddle.net/hsszvoqk/52/