allow nil ns-prefix in codegen/write-namespace!

[conao3]

Closes #211

Allow nil or blank :ns-prefix in codegen/write-namespace!.

Previously, using a nil :ns-prefix resulted in a namespace starting with . (e.g., .numpy), which is an illegal Java class name.

Changes

• Modified write-namespace! to skip the prefix and dot when :ns-prefix is nil or blank
• Added tests for the ns-prefix behavior

Example

;; Before: (write-namespace! "numpy" {:ns-prefix nil})
;; => generates namespace ".numpy" (invalid)

;; After: (write-namespace! "numpy" {:ns-prefix nil})
;; => generates namespace "numpy" (valid)