Try again to improve Evaluator.get performance

Benchmark results from two commits ago versus now:

=== DefaultStackEvaluator - lessThan(Variable, Variable) ===
                         BEFORE                       AFTER
  Type            ns/op          ops/sec       ns/op          ops/sec
  -------------------------------------------------------------------
  Boolean          26.2       38,141,539        19.1       52,356,402
  String           24.7       40,538,160        19.0       52,750,213
  Integer          27.5       36,387,683        27.9       35,805,309
  Long             31.8       31,436,308        33.4       29,953,972
  Float            35.2       28,424,840        43.0       23,264,335
  Double           36.6       27,305,005        45.3       22,061,985
  BigInteger       48.0       20,831,668        55.2       18,115,197
  BigDecimal       53.9       18,564,329        63.2       15,825,842

=== DefaultTreeEvaluator - lessThan(Variable, Variable) ===
                         BEFORE                       AFTER
  Type            ns/op          ops/sec       ns/op          ops/sec
  -------------------------------------------------------------------
  Boolean          41.6       24,057,350        32.9       30,367,349
  String           35.7       28,028,466        27.2       36,753,540
  Integer          45.4       22,031,440        37.9       26,411,280
  Long             48.5       20,603,576        40.8       24,530,858
  Float            52.8       18,935,419        45.6       21,953,894
  Double           59.8       16,733,346        50.1       19,969,427
  BigInteger       75.6       13,223,210        45.9       21,796,485
  BigDecimal       66.4       15,051,269        51.4       19,472,756

Author: ctrueden

src/main/java/org/scijava/parsington/eval/AbstractEvaluator.java

@@ -73,11 +73,13 @@ public void setStrict(final boolean strict) {
 		this.strict = strict;
 	}
 
+	/** Sentinel object to disambiguate between missing vars and null values. */
+	private static final Object MISSING = new Object();
+
 	@Override
 	public Object get(final String name) {
-		final Object value = vars.get(name);
-		// NB: Take care to handle null-valued variables correctly.
-		if (value != null || vars.containsKey(name)) return value;
+		final Object value = vars.getOrDefault(name, MISSING);
+		if (value != MISSING) return value;
 		if (strict) throw new IllegalArgumentException("Unknown variable: " + name);
 		return new Unresolved(name);
 	}