NewScriptPubkey: fix panic on empty scriptPubKey

[starius]

If scriptPubKey is empty, we can't get &scriptPubKey[0] from it.

Also added nix dev shell so I can do nix develop and get all the dependencies loaded.

[stringintech]

Thanks for this and adding test coverage!
Would you mind opening a separate PR for the nix commit so I can give it more thought?
Also for the test, it seems it can be added as a new case in TestInvalidScripts():

		{
			name:            "empty_scriptpubkey",
			scriptPubkeyHex: "",
			amount:          0,
			// Minimal coinbase-style transaction with a single empty scriptSig and zero-value output;
			// used to trigger verification paths.
			txToHex:     "01000000010000000000000000000000000000000000000000000000000000000000000000ffffffff00ffffffff0100000000000000000000000000",
			inputIndex:  0,
			description: "empty scriptPubkey should fail verification",
		},

Next to that, adding a scriptHex :="" case to TestScriptPubkeyFromRaw() would be good.

[starius]

@stringintech I removed Nix to another PR: #4
Added a test case for TestInvalidScripts and TestScriptPubkeyFromRaw.
Please take another look!