Fix getTextContent evaluation to only apply TJ horizontal offsets using numeric items/args

While the array argument to TJ should only contain strings and numbers, other
unfortunate items are found in PDFs in the wild, e.g.:

[(Grandes) 0.0 Tc
-250.0 (Client\350les,) 0.0 Tc
-250.0 (Financements) 0.0 Tc
-250.0 (et) 0.0 Tc
-250.0 (March\351s) ] TJ

getOperatorList already properly ignores any non-string, non-numeric values in
TJ arrays; without this patch to getTextContent, returned text items can have
NaN widths due to calculations being applied to those non-numeric values.

src/core/evaluator.js

@@ -1531,7 +1531,7 @@ var PartialEvaluator = (function PartialEvaluatorClosure() {
               for (var j = 0, jj = items.length; j < jj; j++) {
                 if (typeof items[j] === 'string') {
                   buildTextContentItem(items[j]);
-                } else {
+                } else if (isNum(items[j])) {
                   ensureTextContentItem();
 
                   // PDF Specification 5.3.2 states: