empty 'set' in scmOverrides not handled

[rhubert]

recipes/root.yaml

root: True

checkoutSCM:
   scm: git
   url: http://foo/bar

buildScript: /bin/true
packageScript: /bin/true

default.yaml

scmOverrides:
   -
    match:
      url: "http://foo/bar"
    del: [tag, commit]
    set:

An internal Exception has occured. This should not have happenend.
Please open an issue at https://github.com/BobBuildTool/bob with the following backtrace:
Bob version 0.22.0rc2.dev2+gcb7ebf0f
Traceback (most recent call last):
  File "/home/hubert/projects/bob/pym/bob/scripts.py", line 146, in catchErrors
    ret = fun(*args, **kwargs)
  File "/home/hubert/projects/bob/pym/bob/scripts.py", line 254, in cmd
    ret = cmd(args.args, bobRoot)
  File "/home/hubert/projects/bob/pym/bob/scripts.py", line 59, in __ls
    doLS(*args, **kwargs)
  File "/home/hubert/projects/bob/pym/bob/cmds/misc.py", line 108, in doLS
    for (stack, root) in packages.queryTreePath(args.package, showAlternates):
  File "/home/hubert/projects/bob/pym/bob/pathspec.py", line 910, in queryTreePath
    (nodes, valid) = self.__query(path)
  File "/home/hubert/projects/bob/pym/bob/pathspec.py", line 887, in __query
    root = self.__getGraphRoot()
  File "/home/hubert/projects/bob/pym/bob/pathspec.py", line 838, in __getGraphRoot
    self.__graph = PkgGraphNode.init(".bob-tree.sqlite3", self.__cacheKey,
  File "/home/hubert/projects/bob/pym/bob/pathspec.py", line 592, in init
    root = PkgGraphNode.__convertPackageToGraph(db, rootGenerator())
  File "/home/hubert/projects/bob/pym/bob/pathspec.py", line 901, in getRootPackage
    self.__root = self.__generator()
  File "/home/hubert/projects/bob/pym/bob/input.py", line 3774, in <lambda>
    lambda: self.__generatePackages(nameFormatter, cacheKey, sandboxEnabled),
  File "/home/hubert/projects/bob/pym/bob/input.py", line 3747, in __generatePackages
    result = self.__rootRecipe.prepare(self.__rootEnv, sandboxEnabled, states)[0]
  File "/home/hubert/projects/bob/pym/bob/input.py", line 2390, in prepare
    p, s = r.prepare(thisDepEnv, sandboxEnabled, depStates,
  File "/home/hubert/projects/bob/pym/bob/input.py", line 2565, in prepare
    srcCoreStep = p.createCoreCheckoutStep(self.__checkout,
  File "/home/hubert/projects/bob/pym/bob/input.py", line 1520, in createCoreCheckoutStep
    ret = self.checkoutStep = CoreCheckoutStep(self, checkout, checkoutSCMs,
  File "/home/hubert/projects/bob/pym/bob/input.py", line 1269, in __init__
    self.scmList = [ Scm(scm, fullEnv, overrides, recipeSet)
  File "/home/hubert/projects/bob/pym/bob/input.py", line 1269, in <listcomp>
    self.scmList = [ Scm(scm, fullEnv, overrides, recipeSet)
  File "/home/hubert/projects/bob/pym/bob/input.py", line 387, in Scm
    matched, spec = override.mangle(spec, env)
  File "/home/hubert/projects/bob/pym/bob/scm/scm.py", line 75, in mangle
    rm, set = self.__applyEnv(env)
  File "/home/hubert/projects/bob/pym/bob/scm/scm.py", line 68, in __applyEnv
    for (k,v) in self.__set.items()
AttributeError: 'NoneType' object has no attribute 'items'

[rhubert]

possible fix:

diff --git a/pym/bob/input.py b/pym/bob/input.py
index 31cc3bae..b0b526bb 100644
--- a/pym/bob/input.py
+++ b/pym/bob/input.py
@@ -2883,6 +2883,15 @@ class MountValidator:

         raise schema.SchemaError(None, "Mount entry must be a string or a two/three items list!")

+class DummyValidator:
+    def __init__(self, name):
+        self.__name = name
+
+    def validate(self, data):
+        if data is not None:
+            return data
+        raise schema.SchemaError(None, self.__name + " should not be empty")
+
 class RecipeSet:
     """The RecipeSet corresponds to the project root directory.

@@ -3139,7 +3148,7 @@ class RecipeSet:
                     schema.Optional('if') : schema.Or(str, IfExpression),
                     schema.Optional('match') : schema.Schema({ str: object }),
                     schema.Optional('del') : [str],
-                    schema.Optional('set') : object,
+                    schema.Optional('set') : schema.Schema(DummyValidator("scmOverrides: set")),
                     schema.Optional('replace') : schema.Schema({
                         str : schema.Schema({
                             'pattern' : str,