Fix empty block args bug tap{||}

[tompng]

BlockParametersNode#parameters can be nil for tap{||}. Added nil check for this.

# tap{||}
@ BlockNode (location: (1,3)-(1,7))
├── locals: []
├── parameters:
│   @ BlockParametersNode (location: (1,4)-(1,6))
│   ├── parameters: ∅ # ← This part
│   ├── locals: (length: 0)
│   ├── opening_loc: (1,4)-(1,5) = "|"
│   └── closing_loc: (1,5)-(1,6) = "|"
├── body: ∅
├── opening_loc: (1,3)-(1,4) = "{"
└── closing_loc: (1,6)-(1,7) = "}",

# tap{}
@ BlockNode (location: (1,3)-(1,5))
├── locals: []
├── parameters: ∅
├── body: ∅
├── opening_loc: (1,3)-(1,4) = "{"
└── closing_loc: (1,4)-(1,5) = "}",