Division by zero in HQ, hf_spectrum_sparseness

Possible division by zero in

https://forge.3gpp.org/rep/ivas-codec-pc/ivas-codec/-/blob/main/lib_enc/hq_classifier_enc.c#L615

Observed in BASOP when forcing HQ and the target MDCT spectrum is zero. Could potentially be solved by adding a small number to avoid the zero denominator. The BASOP seems to use Q9 for the inv_rms_fx variable, so adding 2^-9 before division?