[varLib.varStore] Add operator __bool__ and use it

Part of https://github.com/fonttools/fonttools/issues/2211#issuecomment-790125437

Lib/fontTools/feaLib/builder.py

@@ -764,7 +764,7 @@ class Builder(object):
         gdef.Version = 0x00010002 if gdef.MarkGlyphSetsDef else 0x00010000
         if self.varstorebuilder:
             store = self.varstorebuilder.finish()
-            if store.VarData:
+            if store:
                 gdef.Version = 0x00010003
                 gdef.VarStore = store
                 varidx_map = store.optimize()

Lib/fontTools/varLib/__init__.py

@@ -606,7 +606,7 @@ def _add_BASE(font, masterModel, master_ttfs, axisTags):
 	merger.mergeTables(font, master_ttfs, ['BASE'])
 	store = merger.store_builder.finish()
 
-	if not store.VarData:
+	if not store:
 		return
 	base = font['BASE'].table
 	assert base.Version == 0x00010000
@@ -621,7 +621,7 @@ def _merge_OTL(font, model, master_fonts, axisTags):
 
 	merger.mergeTables(font, master_fonts, ['GSUB', 'GDEF', 'GPOS'])
 	store = merger.store_builder.finish()
-	if not store.VarData:
+	if not store:
 		return
 	try:
 		GDEF = font['GDEF'].table

Lib/fontTools/varLib/varStore.py

@@ -135,6 +135,11 @@ def VarRegion_get_support(self, fvar_axes):
 
 ot.VarRegion.get_support = VarRegion_get_support
 
+def VarStore___bool__(self):
+    return bool(self.VarData)
+
+ot.VarStore.__bool__ = VarStore___bool__
+
 class VarStoreInstancer(object):
 
 	def __init__(self, varstore, fvar_axes, location={}):