Fix delete workspace-member avatar (#9630)

## Context
avatarUrl is a TEXT field type so non nullable, which means if we try to
run a mutation with null it will either fail or be ignored. Here this is
the second option, done in sanitizeRecordInput where when a
fieldMetadata has isNullable=false and the value is null, we return
undefined. This caused the mutation to send an empty input and not
remove the avatar

packages/twenty-front/src/modules/ui/input/components/ImageInput.tsx

@@ -172,7 +172,7 @@ export const ImageInput = ({
           />
         </StyledButtonContainer>
         <StyledText>
-          We support your best PNGs, JPEGs and GIFs portraits under 10MB
+          We support your square PNGs, JPEGs and GIFs under 10MB
         </StyledText>
         {errorMessage && <StyledErrorText>{errorMessage}</StyledErrorText>}
       </StyledContent>