Fix array access expression getKtType() bug

For an array access expression e.g., `foo[3]`,
KtFirExpressionTypeProvider first tries
`getReturnTypeForArrayStyleAssignmentTarget(..)` method that determines
the type of the array access expression when it is in the form of "set"
the value. However, the method also returns a non-null result for an
array access expression that reads its value i.e., "get". In this case,
the result of `getKtType()` becomes wrong. This commit lets the method
return a null for the type of "get" array access expression.

analysis/analysis-api/testData/components/expressionTypeProvider/expressionType/assignment/readArrayElement.kt

@@ -0,0 +1,14 @@
+class FOO {
+    var str: String = ""
+    operator fun plusAssign(tk: String?) {
+        str += tk
+    }
+}
+
+fun main(args: Array<String?>) {
+    val foo: FOO = FOO()
+    if (args.size > 2)
+        foo += <expr>args[2]</expr>
+    else
+        foo += foo.toString()
+}

analysis/analysis-api/testData/components/expressionTypeProvider/expressionType/assignment/readArrayElement.txt

@@ -0,0 +1,2 @@
+expression: args[2]
+type: kotlin.String?