ParamIterator: add support for zero parameters: () or <>

The displayed tooltips for macros and identifiers with zero parameters
were correct even before this commit. For example,
"Function macro: mempty()" for `#define mempty()`;
"ClassName< > objectName" for objectName in `ClassName<> objectName;`.
However the unit tests revealed that the corresponding MacroDefinition
object contained a single parameter with an empty name. This didn't pose
any practical problems, but wrong models could cause issues later.

MacroNavigationContext::html() displayed parentheses only if at least
one parameter existed. In order to keep the tooltip correct (as opposed
to "Function macro: mempty" for the example above), display parentheses
if the macro is function-like instead.