Add imports only test, fix instrumenter

lib/instrumenter.js

@@ -70,7 +70,11 @@ class Instrumenter {
 
     // Walk the AST, recording injection points
     ast = SolidityParser.parse(contract.instrumented, {range: true});
-    contract.contractName = ast.children.filter(node => this._isRootNode(node))[0].name;
+
+    const root = ast.children.filter(node => this._isRootNode(node));
+
+    // Handle contracts which only contain import statements
+    contract.contractName = (root.length) ? root[0].name : null;
     parse[ast.type](contract, ast);
 
     // We have to iterate through these points in descending order

test/integration/projects/import-paths/contracts/OnlyUsesImports.sol

@@ -0,0 +1,5 @@
+pragma solidity >=0.4.21 <0.6.0;
+
+import "package/AnotherImport.sol";
+
+interface Void {}