Permit empty keys in templates. Simplifies using functions that don't require a value

src/calibre/ebooks/metadata/book/base.py

@@ -42,6 +42,8 @@ field_metadata = FieldMetadata()
 class SafeFormat(TemplateFormatter):
 
     def get_value(self, orig_key, args, kwargs):
+        if not orig_key:
+            return ''
         key = orig_key.lower()
         if key != 'title_sort' and key not in TOP_LEVEL_IDENTIFIERS:
             key = field_metadata.search_term_to_field_key(key)

src/calibre/library/save_to_disk.py

@@ -134,6 +134,8 @@ class SafeFormat(TemplateFormatter):
     '''
 
     def get_value(self, key, args, kwargs):
+        if key == '':
+            return ''
         try:
             key = key.lower()
             try:

src/calibre/utils/formatter.py

@@ -342,6 +342,8 @@ class EvalFormatter(TemplateFormatter):
     A template formatter that uses a simple dict instead of an mi instance
     '''
     def get_value(self, key, args, kwargs):
+        if key == '':
+            return ''
         key = key.lower()
         return kwargs.get(key, _('No such variable ') + key)