fix(angular-table): handle null and number values with flex-render

[riccardoperra]

@KevinVandy grouping example was broken due to flexRenderDirective not handling null content (e.g. when a cell is grouped)

Here a screenshot of the grouping example after the fix

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[jrgokavalsa]

@riccardoperra, I've observed an issue where the aggregatedCell function is not operating correctly in conjunction with age data. Upon debugging, I found that if aggregatedCell returns a number, no values are displayed. However, when it returns a string, it functions as expected. This behavior suggests that there might be a type-related discrepancy within the renderContent function's.

[riccardoperra]

@jrgokavalsa Thanks I've forgot it. I've added a new check in order to support both string and numbers