Consistently declare nullability `@Contract` for core utilities

[sbrannen]

Overview

Although we have @Contract declarations for various methods in some of our core utilities (such as Assert and StringUtils), we have not yet consistently declared nullability contracts via @Contract for all applicable utility methods.

To provide better nullability support, we should therefore consistently declare method-level nullability contracts for the following core utilities.

• SupplierUtils
• ClassUtils
• CollectionUtils
• FileSystemUtils
• ObjectUtils
• PatternMatchUtils
• ReflectionUtils
• ResourceUtils
• SerializationUtils
• StringUtils
• TypeUtils

Related Issues

• Define nullability @Contract for quote and getFilename in StringUtils #34896
• Improve nullability declarations for return types in [NamedParameter]JdbcOperations #34911

[christophstrobl]

When adding @Contract annotations please also consider switching CollectionUtils.contains(@Nullable Iterator<?> iterator, Object element) to CollectionUtils.contains(@Nullable Iterator<?> iterator, @Nullable Object element).

[sbrannen]

Sure, we'll take that into consideration, @christophstrobl.