Python: Fixes Plan using an empty function in describe and incorrectly setting context variables for next steps. (microsoft#4673)

### Motivation and Context
This PR fixes microsoft#4658 and fixes microsoft#2933.

### Description
The first one basically needed a None check before using
`self._function` in the `Plan.describe` method.
The second one was incorrectly setting the values for variables for the
next step in the plan. The solution for this was suggested by
@amritpal-singh-98, thank you!

### Contribution Checklist

<!-- Before submitting this PR, please make sure: -->

- [X] The code builds clean without any errors or warnings
- [X] The PR follows the [SK Contribution
Guidelines](https://github.com/microsoft/semantic-kernel/blob/main/CONTRIBUTING.md)
and the [pre-submission formatting
script](https://github.com/microsoft/semantic-kernel/blob/main/CONTRIBUTING.md#development-scripts)
raises no violations
- [X] All unit tests pass, and I have added new tests where possible
- [X] I didn't break anyone 😄

---------

Co-authored-by: Evan Mattson <[email protected]>

Author: juliomenendez

python/semantic_kernel/planning/plan.py

@@ -240,8 +240,10 @@ def set_default_skill_collection(
         if self._function is not None:
             self._function.set_default_skill_collection(skills)
 
-    def describe(self) -> FunctionView:
-        return self._function.describe()
+    def describe(self) -> Optional[FunctionView]:
+        if self._function is not None:
+            return self._function.describe()
+        return None
 
     def set_available_functions(self, plan: "Plan", context: SKContext) -> "Plan":
         if len(plan.steps) == 0:
@@ -400,19 +402,17 @@ def get_next_step_variables(self, variables: ContextVariables, step: "Plan") ->
             elif param.name in self._state and (self._state[param.name] is not None and self._state[param.name] != ""):
                 step_variables.set(param.name, self._state[param.name])
 
-        for param_var in step.parameters.variables:
-            if param_var in step_variables:
+        for param_name, param_val in step.parameters.variables.items():
+            if param_name in step_variables:
                 continue
 
-            expanded_value = self.expand_from_variables(variables, param_var)
-            if expanded_value.lower() == param_var.lower():
-                step_variables.set(param_var, step.parameters.variables[param_var])
-            elif param_var in variables:
-                step_variables.set(param_var, variables[param_var])
-            elif param_var in self._state:
-                step_variables.set(param_var, self._state[param_var])
+            if param_name in variables:
+                step_variables.set(param_name, param_val)
+            elif param_name in self._state:
+                step_variables.set(param_name, self._state[param_name])
             else:
-                step_variables.set(param_var, expanded_value)
+                expanded_value = self.expand_from_variables(variables, param_val)
+                step_variables.set(param_name, expanded_value)
 
         for item in variables.variables:
             if item not in step_variables: