fix: prevent panic in isArrayKey when node name is empty string

[mmatur]

Summary

isArrayKey accessed name[0] without checking len(name) > 0 first, causing an index out of range panic when processing a YAML manifest with an empty string map key (e.g. "": value).

Root cause

// before
func isArrayKey(name string) bool {
    return name[0] == '[' && name[len(name)-1] == ']'  // panics when name == ""
}

// after
func isArrayKey(name string) bool {
    return len(name) > 0 && name[0] == '[' && name[len(name)-1] == ']'
}

Test

Added a test case in Test_nodeToRawMap with an empty string key to cover this path.