Fix signatures for cellpath access of records (#9793)

# Description
The same procedure as for #9778 repeated for records.

# User-Facing Changes
Commands that directly supported applying their work directly to record
fields via cell paths, that worked before #9680 will now work again

# Tests + Formatting
Tried to limit the need to add new `.allow_variants_without_examples()`
by adjusting or adding tests to also use some records with access.

crates/nu-command/src/strings/encode_decode/decode_base64.rs

@@ -27,6 +27,7 @@ impl Command for DecodeBase64 {
                     Type::List(Box::new(Type::Binary)),
                 ),
                 (Type::Table(vec![]), Type::Table(vec![])),
+                (Type::Record(vec![]), Type::Record(vec![])),
             ])
             .vectorizes_over_list(true)
             .allow_variants_without_examples(true)

crates/nu-command/src/strings/encode_decode/encode_base64.rs

@@ -33,6 +33,7 @@ impl Command for EncodeBase64 {
                     Type::List(Box::new(Type::String)),
                 ),
                 (Type::Table(vec![]), Type::Table(vec![])),
+                (Type::Record(vec![]), Type::Record(vec![])),
             ])
             .vectorizes_over_list(true)
             .allow_variants_without_examples(true)