From the tutorial (open the template editor and use the Tutorial link at the bottom)
Code:
For example, assume you want to use the template:
{series} - {series_index} - {title}
If the book has no series, the answer will be - - title. Many people would rather the result be simply title, without the hyphens. To do this, use the extended syntax {field:|prefix_text|suffix_text}. When you use this syntax, if field has the value SERIES then the result will be prefix_textSERIESsuffix_text. If field has no value, then the result will be the empty string (nothing); the prefix and suffix are ignored. The prefix and suffix can contain blanks. Do not use subtemplates (`{ … }`) or functions (see below) as the prefix or the suffix.
Using this syntax, we can solve the above series problem with the template:
{series}{series_index:| - | - }{title}
The hyphens will be included only if the book has a series index, which it will have only if it has a series.