Add rule: empty-values, to forbid implicit nulls

only in block mappings for now

yamllint/conf/default.yaml

@@ -32,6 +32,8 @@ rules:
     max: 2
     max-start: 0
     max-end: 0
+  empty-values:
+    forbid-in-block-mappings: false
   hyphens:
     max-spaces-after: 1
   indentation:

yamllint/rules/__init__.py

@@ -24,6 +24,7 @@ from yamllint.rules import (
     document_end,
     document_start,
     empty_lines,
+    empty_values,
     hyphens,
     indentation,
     key_duplicates,
@@ -45,6 +46,7 @@ _RULES = {
     document_end.ID: document_end,
     document_start.ID: document_start,
     empty_lines.ID: empty_lines,
+    empty_values.ID: empty_values,
     hyphens.ID: hyphens,
     indentation.ID: indentation,
     key_duplicates.ID: key_duplicates,

yamllint/rules/empty_values.py

@@ -0,0 +1,69 @@
+# -*- coding: utf-8 -*-
+# Copyright (C) 2017 Greg Dubicki
+#
+# This program is free software: you can redistribute it and/or modify
+# it under the terms of the GNU General Public License as published by
+# the Free Software Foundation, either version 3 of the License, or
+# (at your option) any later version.
+#
+# This program is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+# GNU General Public License for more details.
+#
+# You should have received a copy of the GNU General Public License
+# along with this program.  If not, see <http://www.gnu.org/licenses/>.
+
+"""
+Use this rule to prevent nodes with empty content, that implicitly result in
+``null`` values.
+
+.. rubric:: Options
+
+* Use ``forbid-in-block-mappings`` to prevent implicit empty values in block
+  mappings.
+
+.. rubric:: Examples
+
+#. With ``empty-values: {forbid-in-block-mappings: true}``
+   the following code snippets would **PASS**:
+   ::
+
+    some-mapping:
+      sub-element: correctly indented
+
+   ::
+
+    explicitly-null: null
+
+   the following code snippets would **FAIL**:
+   ::
+
+    some-mapping:
+    sub-element: incorrectly indented
+
+   ::
+
+    implicitly-null:
+
+"""
+
+import yaml
+
+from yamllint.linter import LintProblem
+
+
+ID = 'empty-values'
+TYPE = 'token'
+CONF = {'forbid-in-block-mappings': bool}
+
+
+def check(conf, token, prev, next, nextnext, context):
+
+    if conf['forbid-in-block-mappings']:
+        if isinstance(token, yaml.ValueToken) and isinstance(next, (
+                yaml.KeyToken, yaml.BlockEndToken,
+                yaml.StreamEndToken, yaml.DocumentEndToken)):
+            yield LintProblem(token.start_mark.line + 1,
+                              token.end_mark.column + 1,
+                              'empty value in block mapping')