Resolve get_first clippy lint

    warning: accessing first element with `variant.fields.get(0)`
        --> serde_derive/src/de.rs:1843:27
         |
    1843 |             let default = variant.fields.get(0).map(|field| {
         |                           ^^^^^^^^^^^^^^^^^^^^^ help: try: `variant.fields.first()`
         |
         = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#get_first
         = note: `-W clippy::get-first` implied by `-W clippy::all`
         = help: to override `-W clippy::all` add `#[allow(clippy::get_first)]`

    warning: accessing first element with `variant.fields.get(0)`
        --> serde_derive/src/de.rs:1888:27
         |
    1888 |             let default = variant.fields.get(0).map(|field| {
         |                           ^^^^^^^^^^^^^^^^^^^^^ help: try: `variant.fields.first()`
         |
         = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#get_first

serde_derive/src/de.rs

@@ -1840,7 +1840,7 @@ fn deserialize_internally_tagged_variant(
             let this_value = &params.this_value;
             let type_name = params.type_name();
             let variant_name = variant.ident.to_string();
-            let default = variant.fields.get(0).map(|field| {
+            let default = variant.fields.first().map(|field| {
                 let default = Expr(expr_is_missing(field, cattrs));
                 quote!((#default))
             });
@@ -1885,7 +1885,7 @@ fn deserialize_untagged_variant(
             let this_value = &params.this_value;
             let type_name = params.type_name();
             let variant_name = variant.ident.to_string();
-            let default = variant.fields.get(0).map(|field| {
+            let default = variant.fields.first().map(|field| {
                 let default = Expr(expr_is_missing(field, cattrs));
                 quote!((#default))
             });