[Java.Interop] Fix NRT warnings introduced by targeting 'net6.0' (#840)

In 4d0cba6 we began targeting `net6.0` instead of `netcoreapp3.1`.
This framework contains additional nullable annotations that resulted
in new warnings. Fix those warnings.

Author: jpobst

src/Java.Interop/Java.Interop/JniRuntime.JniTypeManager.cs

@@ -176,7 +176,7 @@ protected virtual IEnumerable<string> GetSimpleReferences (Type type)
 staticreadonlyType[]EmptyTypeArray=Array.Empty<Type>();
 
 
-publicTypeGetType(JniTypeSignaturetypeSignature)
+publicType?GetType(JniTypeSignaturetypeSignature)
 {
 AssertValid();
 

src/Java.Interop/Java.Interop/JniRuntime.JniValueManager.cs

@@ -333,7 +333,7 @@ static Type GetPeerType (Type type)
 returnGetActivationConstructor(fallbackType);
 }
 
-staticConstructorInfoGetActivationConstructor(Typetype)
+staticConstructorInfo?GetActivationConstructor(Typetype)
 {
 return
 (fromcintype.GetConstructors(BindingFlags.Public|BindingFlags.NonPublic|BindingFlags.Instance)
@@ -675,6 +675,8 @@ public override Expression CreateReturnValueFromManagedExpression (JniValueMarsh
 
 publicoverrideExpressionCreateParameterToManagedExpression(JniValueMarshalerContextcontext,ParameterExpressionsourceValue,ParameterAttributessynchronize,Type?targetType)
 {
+targetType??=typeof(object);
+
 varr=Expression.Variable(targetType,sourceValue.Name+"_val");
 context.LocalVariables.Add(r);
 context.CreationStatements.Add(

src/Java.Interop/Java.Interop/JniStringValueMarshaler.cs

@@ -61,6 +61,8 @@ public override Expression CreateParameterToManagedExpression (JniValueMarshaler
 {
 Func<IntPtr,string?>m=JniEnvironment.Strings.ToString;
 
+targetType??=typeof(object);
+
 varvalue=Expression.Variable(targetType,sourceValue.Name+"_val");
 context.LocalVariables.Add(value);
 context.CreationStatements.Add(Expression.Assign(value,Expression.Call(m.GetMethodInfo(),sourceValue)));

src/Java.Interop/Java.Interop/JniValueMarshaler.cs

@@ -15,7 +15,7 @@ sealed class VariableCollection : KeyedCollection<string, ParameterExpression> {
 
 protectedoverridestringGetKeyForItem(ParameterExpressionitem)
 {
-returnitem.Name;
+returnitem.Name!;
 }
 }
 
@@ -169,7 +169,7 @@ public virtual Expression CreateReturnValueFromManagedExpressi
 returnReturnObjectReferenceToJni(context,sourceValue.Name,Expression.Property(s,"ReferenceValue"));
 }
 
-protectedExpressionReturnObjectReferenceToJni(JniValueMarshalerContextcontext,stringnamePrefix,ExpressionsourceValue)
+protectedExpressionReturnObjectReferenceToJni(JniValueMarshalerContextcontext,string?namePrefix,ExpressionsourceValue)
 {
 Func<JniObjectReference,IntPtr>m=JniEnvironment.References.NewReturnToJniRef;
 varr=Expression.Variable(MarshalType,namePrefix+"_rtn");