fix: nil map assignment panics on zero-value Viper struct

[humanborch]

Overview:
Lazily initializes internally mutated maps and key delimiters inside viper.Set and viper.SetDefault methods, enabling safe, immediate usability of an uninitialized zero-value &viper.Viper{} struct without map assignment panics.

What problem does it solve?:

• Fixes a core configuration bug where invoking overriding functions on explicitly constructed zero-value Viper instances (v := &viper.Viper{}) immediately crashes the host application deep inside util.go.
• Pre-handling v.keyDelim initialization prevents unintended character-by-character map splitting on empty delimiter bindings.
• Closes Zero-value Viper struct panics on SetDefault/Set (nil map assignment) #2111

Special notes for a reviewer:
Added test-cases (TestZeroValueViperSetDefault, TestZeroValueViperSet) into the test suite that specifically execute map-binding assignments on the zero-value struct pattern to protect against regression.