dbus-sdr: Support NaN thresholds

It is possible that the threshold interface (Warning or Critical) is
present, but one of its thresholds is not set. In this case IPMI should
report 'na' for this threshold.

Before:
P0 VDD CORE      | na | Amps | na | na | 0.000 | na | na | 49.980 | na

After:
P0 VDD CORE      | na | Amps | na | na | na    | na | na | 49.980 | na

Signed-off-by: Konstantin Aladyshev <aladyshev22@gmail.com>
Change-Id: Ia524d56d0d4944a53973edb8a8a5c34ad848053a
1 file changed