Fix two bugs related to empty magmas

First, SubadditiveMagma(a,[]) computes an empty magma, but we erroneously set
IsTrivial for it, which implies size 1. This was changed to IsEmpty.

Secondly, there was an implication from "IsFiniteOrderElementCollection and
IsMagma" to IsMagmaWithInverses. But such a collection may be empty, hence the
implication is invalid as given.

Fixes #2400

lib/addmagma.gi

@@ -175,7 +175,7 @@ InstallGlobalFunction( SubadditiveMagmaNC, function( M, gens )
     if IsEmpty( gens ) then
       K:= NewType( FamilyObj(M),
                        IsAdditiveMagma
-                   and IsTrivial
+                   and IsEmpty
                    and IsAttributeStoringRep );
       S:= Objectify( K, rec() );
       SetGeneratorsOfAdditiveMagma( S, [] );

lib/magma.gd

@@ -129,8 +129,13 @@ DeclareCategory( "IsMagmaWithInverses",
         IsMagmaWithInversesIfNonzero
     and IsMultiplicativeElementWithInverseCollection );
 
+# FIXME: this is wrong for empty magmas
+# InstallTrueMethod( IsMagmaWithInverses,
+#     IsFiniteOrderElementCollection and IsMagma );
+
 InstallTrueMethod( IsMagmaWithInverses,
-    IsFiniteOrderElementCollection and IsMagma );
+    IsFiniteOrderElementCollection and IsMagmaWithOne );
+
 
 #############################################################################
 ##

tst/testbugfix/2018-05-09-submagma.tst

@@ -0,0 +1,54 @@
+#
+# There was a bug where we marked a sub additive magma
+# with empty generator list as trivial, even though it is empty.
+#
+# There was also a wrong implication, from "IsFiniteOrderElementCollection and
+# IsMagma" to IsMagmaWithInverses. But a collection with the former filters may
+# be empty, and then it isn't a IsMagmaWithInverses.
+#
+gap> amgm:=AdditiveMagma([0]);
+<additive magma with 1 generators>
+gap> Size(amgm);
+1
+gap> IsTrivial(amgm);
+true
+gap> IsEmpty(amgm);
+false
+gap> IsMagmaWithInverses(amgm);
+false
+
+#
+gap> asub:=SubadditiveMagma(amgm, []);
+<additive magma with 0 generators>
+gap> Size(asub);
+0
+gap> IsTrivial(asub);
+false
+gap> IsEmpty(asub);
+true
+gap> IsMagmaWithInverses(asub);
+false
+
+#
+gap> mgm:=Magma( () );
+<commutative semigroup with 1 generator>
+gap> Size(mgm);
+1
+gap> IsTrivial(mgm);
+true
+gap> IsEmpty(mgm);
+false
+gap> IsMagmaWithInverses(mgm);
+false
+
+#
+gap> sub:=Submagma(mgm,[]);
+<commutative semigroup with 0 generators>
+gap> Size(sub);
+0
+gap> IsTrivial(sub);
+false
+gap> IsEmpty(sub);
+true
+gap> IsMagmaWithInverses(sub);
+false