Fix no central orders and completely empty grids in scale variations

[Radonirinaunimi]

This PR fixes two issues arising from specific grids during scale variations, namely:

• grids with no central orders but the corresponding scale varied orders are available
• completely empty grids that do not contain orders

PS: It also bumps the version of pineappl to v1.4.0a1 for the tests.

[Radonirinaunimi]

@kamillaurent Could you please check with this branch (say for one of the datasets and sub-theories that failed before) that it is no longer an issue?

[kamillaurent]

@Radonirinaunimi I checked and seems the problem

ValueError: Central order is not available but sv order is.

is no longer there.

However, for these datasets:

HERA_NC_318GEV_EAVG_BOTTOM-SIGMARED
HERA_NC_318GEV_EAVG_CHARM-SIGMARED
BCDMS_NC_NOTFIXED_P_EM-F2
BCDMS_NC_NOTFIXED_D_EM-F2
NUTEV_CC_NOTFIXED_FE_NB-SIGMARED
NUTEV_CC_NOTFIXED_FE_NU-SIGMARED
CHORUS_CC_NOTFIXED_PB_NU-SIGMARED

I still encounter the second error:

ValueError: min() iterable argument is empty

[Radonirinaunimi]

Could you please try now @kamillaurent ? (Fingers crossed 🤞🏼 it won't break the nFONLL combinations)