Model.var_by_name throws error if the model contains no variables when using gurobi as solver

**Describe the bug**

The behavior of the function `model.get_var_by_name()` is unexpected when using Gurobi as solver. According to the docstring, the function should return None if no variable with that name exists. However, when using it with Gurobi, I get an error `ParameterNotAvailable` when using the function on a model **without any variables**.

**To Reproduce**
1. Create an instance of a model with Gurobi as solver
2. Use the `model.var_by_name(name="randomname")` function.

```
model = mip.Model(solver_name=mip.GUROBI)
variable = model.var_by_name(name="variablename")
```

**Expected behavior**
From the docstring, None should be returned.

**Desktop (please complete the following information):**
- Operating System, version: WSL2 on Windows 11
- Python version: 3.13.11
- Python-MIP version (we recommend you to test with the latest version):

**Additional context**
The behavior of Gurobi is different than for example HiGHS here, which does not throw the error. In my opinion the behavior of `Model.var_by_name()` should not be solver dependent.

The exact same problem exists for the constraint version of this function, namely `model.constrs_by_name()`