types: Prevent panic with uninitialized Number Value (#200)

Reference: #89
Reference: hashicorp/terraform-plugin-go#114

This change will ensure that if the `Number` `Value` pointer is `nil`, that the returned value is an untyped `nil`, rather than a `*big.Float` typed `nil`.

The upstream `(tftypes.Value).IsNull()` method can only detect untyped `nil` for values. A related fix for `tftypes.NewValue()` to convert a `*big.Float` `nil` value into an untyped `nil` has also been merged upstream to fix this in depth.

Previously, this new test case would pass:

```go
		"value-nil": {
			input:       Number{Value: nil},
			expectation: (*big.Float)(nil),
		},
```

Now it appropriately expects:

```go
		"value-nil": {
			input:       Number{Value: nil},
			expectation: nil,
		},
```

.changelog/200.txt

@@ -0,0 +1,3 @@
+```release-note:bug
+types: Prevent panic with uninitialized `Number` `Value`
+```

types/number.go

@@ -55,6 +55,9 @@ func (n Number) ToTerraformValue(_ context.Context) (interface{}, error) {
 	if n.Unknown {
 		return tftypes.UnknownValue, nil
 	}
+	if n.Value == nil {
+		return nil, nil
+	}
 	return n.Value, nil
 }
 

types/number_test.go

@@ -92,6 +92,10 @@ func TestNumberToTerraformValue(t *testing.T) {
 			input:       Number{Value: big.NewFloat(123)},
 			expectation: big.NewFloat(123),
 		},
+		"value-nil": {
+			input:       Number{Value: nil},
+			expectation: nil,
+		},
 		"unknown": {
 			input:       Number{Unknown: true},
 			expectation: tftypes.UnknownValue,