Fix handling of library(name_prefix: [])

Adjust the handling of a name_prefix: [] kwarg to be the same as
name_suffix: [] kwarg, i.e. identically to the case where it's omitted,
so BuildTarget.prefix doesn't get set (so the default is used).

Also clarify the error reported when a non-empty list is used.

mesonbuild/build.py

@@ -922,16 +922,17 @@ This will become a hard error in a future Meson release.''')
             name_prefix = kwargs['name_prefix']
             if isinstance(name_prefix, list):
                 if name_prefix:
-                    raise InvalidArguments('name_prefix array must be empty to signify null.')
+                    raise InvalidArguments('name_prefix array must be empty to signify default.')
-            elif not isinstance(name_prefix, str):
+            else:
-                raise InvalidArguments('name_prefix must be a string.')
+                if not isinstance(name_prefix, str):
-            self.prefix = name_prefix
+                    raise InvalidArguments('name_prefix must be a string.')
-            self.name_prefix_set = True
+                self.prefix = name_prefix
+                self.name_prefix_set = True
         if 'name_suffix' in kwargs:
             name_suffix = kwargs['name_suffix']
             if isinstance(name_suffix, list):
                 if name_suffix:
-                    raise InvalidArguments('name_suffix array must be empty to signify null.')
+                    raise InvalidArguments('name_suffix array must be empty to signify default.')
             else:
                 if not isinstance(name_suffix, str):
                     raise InvalidArguments('name_suffix must be a string.')