Make --pretty-json work with Standard JSON output

test/cmdlineTests/pretty_json_combined/args

@@ -0,0 +1 @@
+--combined-json abi --pretty-json --json-indent 3

test/cmdlineTests/pretty_json_combined/input.sol

@@ -0,0 +1,3 @@
+// SPDX-License-Identifier: GPL-3.0
+pragma solidity >=0.0;
+contract C {}

test/cmdlineTests/pretty_json_combined/output

@@ -0,0 +1,10 @@
+{
+   "contracts":
+   {
+      "pretty_json_combined/input.sol:C":
+      {
+         "abi": []
+      }
+   },
+   "version": "<VERSION REMOVED>"
+}