[C#] For public APIs, check nulls for parameters.

For example,
```
public DataFrameWriter Format(string source)
```
source should be checked if it is null before sending it to JVM side.

One thing to consider is whether we want to check it at the API level, or at the JVMBridge level.

Checking at the API level will introduce lots of code, but will be better for the user since the exception stack will be smaller. Checking at the JVMBridge level will be much less code, but user will see the exception stack from the JVMBridge.