fix: properly handle boolean values in npmrc keys (#1445)

#1407 introduced a bug where the replacement logic breaks if the value of the npmrc key is a boolean.

lib/manager/npm/registry.js

@@ -48,6 +48,11 @@ function setNpmrc(input, exposeEnv = false) {
 }
 
 function envReplace(value, env = process.env) {
+  // istanbul ignore if
+  if (typeof value !== 'string' || !value) {
+    return value;
+  }
+
   const ENV_EXPR = /(\\*)\$\{([^}]+)\}/g;
 
   return value.replace(ENV_EXPR, (match, esc, envVarName) => {