template: Add is_empty method.

This makes it easy to detect when a user has specifically cleared a
template; i.e., from a specific configuration section.

import2ledger/template.py

@@ -260,3 +260,6 @@ class Template:
             if key.endswith('_date'):
                 template_vars[key] = value.strftime(self.date_fmt)
         return ''.join(f(template_vars) for f in self.format_funcs)
+
+    def is_empty(self):
+        return not self.format_funcs

tests/test_templates.py

@@ -26,6 +26,7 @@ def assert_easy_render(tmpl, entity, amount, currency, expect_date, expect_amt):
         "  Accrued:Accounts Receivable  " + expect_amt,
         "  Income:Donations  " + expect_amt.replace(amount, "-" + amount),
     ]
+    assert not tmpl.is_empty()
 
 def test_easy_template():
     tmpl = template_from('Simplest')
@@ -39,6 +40,11 @@ def test_currency_formatting():
     tmpl = template_from('Simplest', signed_currencies=['USD'])
     assert_easy_render(tmpl, 'CC', '7.99', 'USD', '2015/03/14', '$7.99')
 
+def test_empty_template():
+    tmpl = template.Template("\n \n")
+    assert tmpl.render('BB', decimal.Decimal('8.99'), 'USD', DATE) == ''
+    assert tmpl.is_empty()
+
 def test_complex_template():
     template_vars = {
         'entity': 'T-T',