Small updates to `applyRtc()`

[gshiroma]

This PR provides two fixes to applyRtc():

• Consider zero values valid preventing applyRtc() to convert them to NaN.
• Apply minimum RTC area normalization factor (ANF) threshold using rtc_min_value (in linear) rather than rtc_min_value_db (in dB).

Note: these fixes do not affect NISAR or OPERA production.