wrap: correct return annotation of `Resolver.get_from_wrapdb()`

It returns `None | PackageDefinition`, not `PackageDefinition`

mesonbuild/wrap/wrap.py

@@ -351,7 +351,7 @@ class Resolver:
             self.wrapdb_provided_deps.update({i: name for i in info.get('dependency_names', [])})
             self.wrapdb_provided_programs.update({i: name for i in info.get('program_names', [])})
 
-    def get_from_wrapdb(self, subp_name: str) -> PackageDefinition:
+    def get_from_wrapdb(self, subp_name: str) -> T.Optional[PackageDefinition]:
         info = self.wrapdb.get(subp_name)
         if not info:
             return None
@@ -405,11 +405,10 @@ class Resolver:
         self.packagename = packagename
         self.directory = packagename
         wrap = self.wraps.get(packagename)
-        if not wrap:
+        if wrap is None:
             wrap = self.get_from_wrapdb(packagename)
-        if not wrap:
-            m = f'Neither a subproject directory nor a {self.packagename}.wrap file was found.'
-            raise WrapNotFoundException(m)
+            if wrap is None:
+                raise WrapNotFoundException(f'Neither a subproject directory nor a {self.packagename}.wrap file was found.')
         self.wrap = wrap
         self.directory = self.wrap.directory