Load Variable does not work properly for integers

**Describe the bug**
When you try to load an integer variable, and the key doesn't exist, the variable gets set to 0 instead of being unmodified.

**To Reproduce**
1. Create a new scene
2. Create a flowchart in there with an integer variable set to some number other than 0
3. Have a Game Started block that tries to load the integer variable with some key that wasn't registered
4. See as the variable gets set to 0

**Expected behavior**
The variable to be unmodified, as the CommandInfo says.

**Versions & Platform**
 - Unity version: 2019.4.9f1
 - Fungus Version: 3.13
 - Editor OS: Windows 10
 - Build Platform: Desktop Standalone

**Additional context**
I could fix this myself and submit a PR, but rebasing the develop branch might undo the work I've done on another PR I have pending. I suggest changing OnEnter like so:

```
if (key == "" || // Old code. Remove this
            variable == null)
{
    Continue();
    return;
}

// The following should fix the problem
var flowchart = GetFlowchart(); // This chunk was moved from a bit later in OnEnter.
// Prepend the current save profile (if any)
string prefsKey = SetSaveProfile.SaveProfile + "_" + flowchart.SubstituteVariables(key);
if (key == "" || variable == null || !PlayerPrefs.HasKey(prefsKey)) 
{
    Continue();
    return;
}
```
