In some cases a complex value can be an empty object, e.g. if a complex type only contains navigation properties, or only its navigation properties are specified in $select, and the response format is minimal JSON:

• 7.2: A complex value is represented as a single JSON object containing one name/value pair for each property that makes up the complex type.
• 8.1: The navigation link for a navigation property is only represented if the client requests odata.metadata=full or the navigation link cannot be computed[...]

So it clearly is a JSON object with zero name-value pairs (one for each property = zero, one for each non-computable navigation = zero), but it is somewhat hard to deduce this from the specification text. An explicit statement and/or example would help.

Another example is an open complex type that for this instance doesn't have any properties.