[forms][bugfix] Fix GeometryType in QuadratureRuleKey exported by Forms::Coefficient

So far a Forms::Coefficient exported a QuadratureRuleKey with GeometryType being none with appropriate dimension. This works when assembling linear and bilinear forms, because we always multiply with at least one unary operator which then sets the correct GeometryType.

However, if we want to use the nullary operator Forms::Coefficient standalone, e.g. to simply integrate it (it's a grid function after all), we silently use the wrong GeometryType. Luckily we can always derive the correct GeometryType from the element this function is bound to allowing for an easy fix.