Allow null for the SANE source resolution field.

Signed-off-by: Roberto Rosario <roberto.rosario.gonzalez@gmail.com>

mayan/apps/sources/migrations/0016_auto_20170630_2040.py

@@ -0,0 +1,20 @@
+# -*- coding: utf-8 -*-
+# Generated by Django 1.10.7 on 2017-06-30 20:40
+from __future__ import unicode_literals
+
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('sources', '0015_auto_20170206_0835'),
+    ]
+
+    operations = [
+        migrations.AlterField(
+            model_name='sanescanner',
+            name='resolution',
+            field=models.PositiveIntegerField(blank=True, help_text='Sets the resolution of the scanned image in DPI (dots per inch). Typical value is 200. If this option is not supported by your scanner, leave it blank.', null=True, verbose_name='Resolution'),
+        ),
+    ]

mayan/apps/sources/models.py

@@ -185,7 +185,7 @@ class SaneScanner(InteractiveSource):
         ), max_length=16, verbose_name=_('Mode')
     )
     resolution = models.PositiveIntegerField(
-        blank=True, help_text=_(
+        blank=True, null=True, help_text=_(
             'Sets the resolution of the scanned image in DPI (dots per inch). '
             'Typical value is 200. If this option is not supported by your '
             'scanner, leave it blank.'