Times should have always at least 2 elements

[mmatera]

This PR fixes a tiny inconsistency. In WMA, pHoldForm[Times[expr]] produces as a formatted output Times[expr]. Our current format_times function hides this, but the issue is revealed if we rewrite it to make it closer to the WMA routine.

[rocky]

I tried in the master branch and in this branch and in both cases I see:

In[1]:= HoldForm[Times[expr]]
Out[1]= expr

(I do see on WMA that we get Times[expr] for Out[1].)

[mmatera]

This is what you get in WMA:

In[1]:= HoldForm[Times[expr]]                                                   

Out[1]= Times[expr]

In[2]:= HoldForm[Times[]]                                                       

Out[2]= Times[]

and this is what this PR tries to fix.