Make checks a bit more robust against empty entries.

mesonbuild/build.py

@@ -1637,6 +1637,10 @@ class CustomTarget(Target):
         for i in self.outputs:
             if not(isinstance(i, str)):
                 raise InvalidArguments('Output argument not a string.')
+            if i == '':
+                raise InvalidArguments('Output must not be empty.')
+            if i.strip() == '':
+                raise InvalidArguments('Output must not consist only of whitespace.')
             if '/' in i:
                 raise InvalidArguments('Output must not contain a path segment.')
             if '@INPUT@' in i or '@INPUT0@' in i:

mesonbuild/interpreter.py

@@ -3006,6 +3006,8 @@ different subdirectory.
     def add_target(self, name, tobj):
         if name == '':
             raise InterpreterException('Target name must not be empty.')
+        if name.strip() == '':
+            raise InterpreterException('Target name must not consist only of whitespace.')
         if name.startswith('meson-'):
             raise InvalidArguments("Target names starting with 'meson-' are reserved "
                                    "for Meson's internal use. Please rename.")