WIP: disallow 'nil' for strings and seqs

compiler/ast.nim

@@ -1150,7 +1150,10 @@ proc copyObjectSet*(dest: var TObjectSet, src: TObjectSet) =
   for i in countup(0, high(src.data)): dest.data[i] = src.data[i]
 
 proc discardSons*(father: PNode) =
-  father.sons = nil
+  when defined(nimNoNilSeqs):
+    father.sons = @[]
+  else:
+    father.sons = nil
 
 proc withInfo*(n: PNode, info: TLineInfo): PNode =
   n.info = info
@@ -1367,7 +1370,7 @@ proc createModuleAlias*(s: PSym, newIdent: PIdent, info: TLineInfo;
   result.loc = s.loc
   result.annex = s.annex
   # XXX once usedGenerics is used, ensure module aliases keep working!
-  assert s.usedGenerics == nil
+  assert s.usedGenerics.len == 0
 
 proc initStrTable*(x: var TStrTable) =
   x.counter = 0
@@ -1592,7 +1595,10 @@ proc getStr*(a: PNode): string =
   of nkStrLit..nkTripleStrLit: result = a.strVal
   of nkNilLit:
     # let's hope this fixes more problems than it creates:
-    result = nil
+    when defined(nimNoNilSeqs):
+      result = ""
+    else:
+      result = nil
   else:
     doAssert false, "getStr"
     #internalError(a.info, "getStr")