Division by zero in fields

[ntessore]

The mean and bias calculation in fields contains potential divisions by zero when the catalogue is empty:

# Positions

        # normalize map
        pos /= nbar

        # compute bias of number counts
        bias = ngal / (4 * np.pi) * mapper.area**2 / nbar**2

# ScalarField

        # normalise the map
        val /= wbar

        # compute bias from variance (per object)
        bias = 4 * np.pi * vbar**2 * (var / wmean**2) / ngal

# ComplexField

        # normalise the map
        val /= wbar

        # bias from measured variance, for E/B decomposition
        bias = 2 * np.pi * vbar**2 * (var / wmean**2) / ngal

# Weights

        # normalise the map
        wht /= wbar

        # bias from weights
        bias = 4 * np.pi * vbar**2 * (w2mean / wmean**2) / ngal