Use constant-first equals in QueryTransformers

[namest504]

#4047

Refactors QueryTransformers to use constant-first equals() calls. This is a common best practice to prevent potential NullPointerExceptions if the variable (token) were to be null.

class QueryTokens {
    // ...
    static final QueryToken TOKEN_COMMA = token(", ");
    static final QueryToken TOKEN_AS = expression("AS");
    // ...
}

Before:

if (token.equals(TOKEN_AS)) {
     // ...
 }

After:

if (TOKEN_AS.equals(token)) {
    // ...
}

• You have read the Spring Data contribution guidelines.
• You use the code formatters provided here and have them applied to your changes. Don’t submit any formatting related changes.
• You submit test cases (unit or integration tests) that back your changes.
• You added yourself as author in the headers of the classes you touched. Amend the date range in the Apache license header if needed. For new types, add the license header (copy from another file and set the current year only).

[mp911de]

This is a common best practice to prevent potential NullPointerExceptions if the variable (token) were to be null.

We have proper indication of nullable parameters and whether collections contain nullable elements hence we don't need to address an aspect that isn't broken.