Add check for ambiguous nullary test

Given an input like `if [[ $(a) ]]; then ...`, this is a implicit `-n` test,
so it works like `if [[ -n $(a) ]]; then ...`. Users might confuse this for
a check for the exit code of the command a, which should be tested with:

    if a; then
        ...

We warn the user to be more explicity and specifity the `-n`.

Fixes #1416

src/ShellCheck/Analytics.hs

@@ -170,6 +170,7 @@ nodeChecks = [
     ,checkSubshelledTests
     ,checkInvertedStringTest
     ,checkRedirectionToCommand
+    ,checkNullaryExpansionTest
     ]
 
 
@@ -3096,5 +3097,21 @@ checkRedirectionToCommand _ t =
                 warn id 2238 "Redirecting to/from command name instead of file. Did you want pipes/xargs (or quote to ignore)?"
         _ -> return ()
 
+prop_checkNullaryExpansionTest1 = verify checkNullaryExpansionTest "[[ $(a) ]]"
+prop_checkNullaryExpansionTest2 = verify checkNullaryExpansionTest "[[ $a ]]"
+prop_checkNullaryExpansionTest3 = verifyNot checkNullaryExpansionTest "[[ $a=1 ]]"
+prop_checkNullaryExpansionTest4 = verifyNot checkNullaryExpansionTest "[[ -n $(a) ]]"
+checkNullaryExpansionTest _ t =
+    case t of
+        TC_Nullary _ _ (T_NormalWord id [T_DollarExpansion _ [T_Pipeline _ [] [x]]]) ->
+            when (isJust (getCommand x)) $
+                style id 2243 (
+                "To check for the exit code of a command, remove the conditional expression, e.g. if foo; ...")
+        TC_Nullary _ _ (T_NormalWord id [t2]) ->
+                when ((not . isConstant) t2) $
+                    style id 2244 (
+                    "Use -n to check for null string, e.g. [[ -n $var ]].")
+        _ -> return ()
+
 return []
 runTests =  $( [| $(forAllProperties) (quickCheckWithResult (stdArgs { maxSuccess = 1 }) ) |])