Allow empty templates.

src/TDTemplateEngine.m

@@ -163,7 +163,7 @@ - (TDNode *)compileTemplateFile:(NSString *)path encoding:(NSStringEncoding)enc
 
 
 - (TDNode *)compileTemplateString:(NSString *)str error:(NSError **)err {
-    NSParameterAssert([str length]);
+    NSParameterAssert(str != nil);
     TDAssert([_printStartDelimiter length]);
     TDAssert([_printEndDelimiter length]);
     TDAssert([_tagStartDelimiter length]);

src/TDTemplateParser.m

@@ -75,6 +75,12 @@ - (void)template_ {
     self.currentParent = root;
 
     }];
+
+    if ([self predicts:TOKEN_KIND_BUILTIN_EOF, 0]) {
+        [self empty_];
+        return;
+    }
+
     do {
         [self content_]; 
     } while ([self predicts:TOKEN_KIND_BUILTIN_ANY, 0]);
@@ -204,4 +210,18 @@ - (void)text_ {
 
 }
 
+- (void)empty_ {
+
+    [self matchEOF:NO];
+
+    [self execute:^{
+
+    PKToken *tok = [PKToken tokenWithTokenType:PKTokenTypeWord stringValue:@"" doubleValue:0.0];
+    TDNode *txtNode = [TDTextNode nodeWithToken:tok parent:_currentParent];
+    [_currentParent addChild:txtNode];
+
+    }];
+
+}
+
 @end

src/TDTextNode.m

@@ -34,7 +34,7 @@ - (void)dealloc {
 - (void)renderInContext:(TDTemplateContext *)ctx {
     NSParameterAssert(ctx);
 
-    TDAssert([self.token.stringValue length]);
+    TDAssert(self.token.stringValue != nil);
 
     [ctx writeString:self.token.stringValue];
 }

test/TDPrintNodeTests.m

@@ -34,6 +34,18 @@ - (NSString *)outputString {
     return str;
 }
 
+- (void)testEmptyPassthru {
+    NSString *input = @"";
+    id vars = nil;
+
+    NSError *err = nil;
+    BOOL success = [_engine processTemplateString:input withVariables:vars toStream:_output error:&err];
+    TDTrue(success);
+    TDNil(err);
+    NSString *res = [self outputString];
+    TDEqualObjects(@"", res);
+}
+
 - (void)testAsciiPassthru {
     NSString *input = @"foo";
     id vars = nil;